When the games are soft, and of sites with global player pools that will depend on, what time zone you are in. Most recreational players are from Europe or North and South America. So an excellent time to play is late friday or saturday evening in those time zones, because then some of the players will have been up all night partying.
But really its often more important to play, when you are fresh and feeling the best. So for instance not if you are tired, sleep deprivated, tilted from losing in your last session, depressed, drunk or hungover.