Research has found that the biggest movements in four of the major currency pairs - namely EUR/USD, GBP/USD, USD\JPY and USD/CHF - are observed on Tuesdays and Wednesdays. These days generally show the biggest movements in currencies as major data releases relat-ed to the economy are usually released on these days.
Fridays are busy as well, but only until 12am. During the second half of Friday, movements can often be unpredictable since major banks, hedge funds and financial institutions will close some of their positions for safety reasons over the weekend.