The PM gold fix occurs at 3 pm London time (8 am MST) and the AM gold fix occurs at 10:30 am London time (3:30 am MST) 
from Zero Hedge:
Back in August 2010, we presented an idea proposed by our friends at SK Options trading for a very simple trading strategy: being long gold in the overnight session, and shorting it during the day.  At the time of writing, such a strategy would have returned $2.16  billion from a $100 million initial investment in 10 years, a 37.46%  annualized return. Today, we provide a much needed follow up to this  quite stunning divergence. As SK notes: "we have revisited the article  and written an update. Not only does the discrepancy still exist but it has been actually increasing. That  fund would now be worth $5.26B, way up from $2.16B when we last wrote  about it - in other words an increase of 143% in just over a year. When  we wrote about this in August 2010, the annualized return of the Long  Overnight/Short Intraday gold index was 37.46% since the start of 2001.  However if we measure from now the annualized return since 2001 is  43.24%, with the annualized return of the Long Overnight/Short Intraday gold index standing at roughly 64.4% since 2009."  So for those who wish to layer on an additional alpha buffer on top of  what is already the best performing asset of the past decade, the SK  Options way just may be the strategy. As for the reasons for this gross  arbitrage - who cares. Is it manipulation? is it the early Asian buying  offset by London pool selling? It is largely irrelvant - the point is  that this is "the divergence that keeps on giving" - kinda like a  Stolper trade, or an inverse Tilson ETF, and until it doesn't, or until  something dramatically changes in the precious metal market, it is  likely that this trading pattern will continue for a long time.
From SK Options trading
Revisiting Our Proposal For An Overnight Gold Fund

In August 2010 we wrote an article entitled “Proposing An Overnight Gold Fund”   in which we explored the potential for launching a fund that held long   positions in gold overnight and was short gold during the day. We   pointed out that “a hedge fund starting in 2001 with $100m, with the   strategy of being long gold from the PM to AM fix, and short gold from   the AM to PM fix...would be worth $2.16billion today, before any fees   and expenses.” We have been monitoring this trading strategy since then   and therefore would like to take this opportunity to update readers on   its astonishing progress.
Firstly we will introduce the thinking that led us to investigate   this trading strategy. There is much debate within the precious metals   industry regarding the alleged suppression, or at least manipulation to   an extent, by either central banks or the proprietary trading divisions   of large banks, or a combination of the two.
In April 2010 the US Commodity Futures Trading Commission CFTC fined   Hedge Fund Moore Capital for manipulation of the New York platinum and   palladium futures market, as the firm was found to be “banging the   close”, which involves entering orders in a manner designed to inflate   the closing price, which other various derivatives contracts could be   based on. So that is irrefutable evidence that the precious metals   futures market is, at least to some extent, being manipulated. However a   large concentration of this debate is based not on platinum and   palladium, but on gold and silver, and particularly gold.
There are other theories that could explain this discrepancy that do   not involve manipulation. For example one could take the view that   Eastern market participants are perhaps more bullish on gold than their   Western trading counterparts. Therefore gold is perhaps more likely to   rise during Asian trading and fall when the west takes over.
Numerous hypothesises have been put forward as to the motive behind   alleged suppression of the gold, ranging from a central bank conspiracy   to keep gold prices low, to large trading banks simply exploiting their   market dominance for easy profits, or even a combination of the two  with  the central banks and large bullion trading operations working  together  in some kind for cartel to keep gold prices low. This article  does not  intend to discuss the merits of these theories, however  plausible or  implausible various parties believe them to be. Instead we  will focus on  finding out if a discrepancy exists and if it does, can  one take  advantage of it and use it for profitable trading strategies.
We would like to recommend an excellent article by Adrian Douglas,   editor of Market Force Analysis and a GATA board member entitled “Gold   Market is not “Fixed”, it’s Rigged” which goes into great detail on the   statistics behind the difference between how gold trades between the AM   and PM fix, and how it trades from the PM to AM fix. The very fact  that  there appears to be a significance difference sets our alarm bells   ringing. Whether gold trades in New York, London, Tokyo or Timbuktu,   gold is still gold and so one would expect that it would trade in a   similar fashion across these timeframes over a long period of time.
If we take the change in the gold price from the London AM to PM fix   (intraday gold) compare it to the change in the gold price from the PM   to AM fix (overnight gold), we can see the startling difference between   the two periods of trading. We will demonstrate this by showing what   would have happened if one had theoretically invested in the intraday   gold market from 2001 to present.
Starting in 2001 with an indexed based at 100, the chart below shows   what would have happened to that investment of 100 if it had been used   to purchase gold at the AM fix and sell gold at the PM fix, replicating   the daily percentage performance of gold in the intraday market.

As the chart above shows, the performance is dismal. For example a   hypothetical gold investment fund starting with $100m in 2001, and using   it to buy gold at the AM fix and sell it at the PM fix would now be   left with just $31 million, almost a 70% loss in just under ten years.   Over the same time period gold prices have risen over 590%.
From this we can infer that in fact it was possible to make money   shorting gold everyday for the last decade or so. If a hedge fund or   even an individual trader were to have sold gold at the AM fix and   covered that short position at the PM fix, for each day of this terrific   bull market run in gold, that fund would have almost tripled their   starting capital.

This appears to be a remarkable result, as one would presume that   shorting gold everyday during a period where the yellow metal has risen   590% would have devastated any portfolio, not caused a 178.7% increase.   Those who do not believe in theories of gold price suppression, often   cite the fact that gold prices are at an all time high as a major piece   of evidence to discredit any suggestions of price suppression. After  all  how can the price be being suppressed if prices are sky rocketing?
Well the answer to that question is that if the gold traders at the   large banks accused of such manipulation are just trading during the   intraday market between the AM to PM fix, they may not be too concerned   about how gold trades overnight (provided they are not holding  positions  overnight of course). What matters is how gold trades during  this  intraday period, and if more often than not gold is falling during  this  time, and more often than not the banks are short gold during  this  period, then they are making money regardless of the overnight  price  action.
It would appear that subtle manipulation is more likely that blatant price suppression.
So the question on the mind of many gold bulls might be; how do I   remove this downward manipulation during the intraday period? Even if I   do not believe in manipulation, suppression or any other conspiracy   theories, how do I eliminate this statistical fact that gold is   underperforming during the intraday period?
The answer is to buy gold at the PM fix and sell it the following day   at the AM fix, or more simply put, just be long gold overnight.

The graph above shows how rewarding this strategy would have been,   with a return of 1797% in eleven years, a return 3.2 times greater than   the 590% that would have been made simply buying gold in 2001 holding   until now. With many investors and traders looking for the best way to   lever their gold returns, from pouring over drill results to identify   the best gold stocks to experimenting with leveraged gold ETFs and ETNs,   a more simple solution could be simply to only have long exposure to   gold overnight.
For the more cavalier traders, going long gold overnight and then   short gold for the intraday period, makes for an even more profitable   strategy.

Consider a hedge fund starting in 2001 with $100m, with the strategy   of being long gold from the PM to AM fix, and short gold from the AM to   PM fix. That hedge fund would be worth $5.26billion today, before any   fees and expenses. This should be enough to catch any investor’s   attention. Even without shorting gold during the intraday period,   limiting exposure to gold to just the overnight period enhances returns   enough to justify using this as a basis for a trading strategy.
As stated at the beginning of this article, our focus is not what or   who is causing this discrepancy nor any potential motives for such a   discrepancy, but what action to take in order to profit from it.
What has surprised us most in our ongoing investigation into this   area is that not only is the discrepancy persisting, but it is arguably   increasing. When we first wrote about this in August 2010, the   annualized return of the Long Overnight/Short Intraday gold index was   37.46% since the start of 2001. However if we measure from now the   annualized return since 2001 is 43.24%. the chart below demonstrates   this point, with the annualized return of the Long Overnight/Short   Intraday gold index standing at roughly 64.4% since 2009.

Another point of interest is when this outperformance is   concentrated. The performance around the September 2011 correction is   particularly remarkable. Whilst gold prices plummeted, the Long   Overnight/Short Intraday gold index increased dramatically, having   already been increasing whilst gold rallied over the previous couple of   months.

From this we can infer that the majority of gold’s declines in the   recent major correction occurred during the intraday trading session,   not the overnight trading session.
However in practice we must keep in mind that reversing one’s   position each day is not free. One would have to cross the bid/ask   spread. Taking a $0.10 spread into account the short intraday and long   overnight index would have increased from 100 to 1827.34 since 2001.   This increase of 1727.4% outperforms the 593% increase in gold prices   over the same period by almost 3 times. If a $0.20 spread is used on a   short intraday and long overnight index, there is an increase of 530.4%,   which slightly underperforms a buy and hold strategy. Therefore one   would need to be able to reverse one’s position at the AM and PM fix for   $0.10 spread for the strategy to work in practice.
Nonetheless we still think that this is an important discrepancy that   should be taken into account when trading gold. Even if one does not   explicitly execute this exact trading strategy, one can still benefit   from the trading patterns it is based on. For example if one was nervous   about a correction in gold prices but did not want to be short gold,  it  would perhaps be preferable to close any long position prior to the   intraday trading period and reopen them after the PM fix.
In addition to incorporating these patterns into our trading strategy   at SK Options Trading, we are also looking into the feasibility of   launching some form of investment fund to take advantage of the   opportunities discussed in this article. As part of this feasibility   study we are looking to gauge investor interest and so would welcome any   comments, suggestions or ideas that people may wish to contribute,   simply email info@skoptionstrading.com.