Freerolls are always very soft, so it does not matter, when you play them. However if you are going to play
poker games, where people buy in and play for actual money, then in my experience the games tend to be softer in the early european morning hours until around 7am central european time.
This is because its evening in north and south America, so recreational players from particularly
canada is playing at this time, while many eastern european regulars are sleeping. Volume is also not so high, which might be a further reason, why many regulars dont bother playing.
The worst time to play starts directly after this and is the european daytime hours. Now the canadian recreational players are sleeping, and those in Europe are working, if its a normal weekday leaving mostly regulars to play against each other. The european evening hours are somewhere in between. Volume tend to be highest at this time, and recreational players from Europe are typically playing, but so are most of the regulars.
This is just a generalization, and for cash games in particular its always more important to find the good tables, than when you play. If you can get position on a huge fish, it does not matter, what time of day it is, or how the other tables at the site play.