Here we can see the volumes for the EURUSD currency pair from August 2011 to August 2012
You can clearly see the holiday / low volume periods marked by the yellow areas. These are the times that you do not want to be trading as the lower volumes means that the markets are lacking in liquidity.
This is normally caused by the big institutional traders being on holiday.
This causes even a small news rumour to spike the prices. It also means that the normal price levels for buying and selling at will not have the orders sitting there , so prices tend to break these levels quite easily at these times.
If we look at the blue area this shows use the best months to trade as the volumes are higher in these months.
We are now into September and as can be seen last year the volumes increased into the normal levels around the beginning of the second full trading week.
Trade safely !