Friday and Saturday between, say, 0:00 and 10:00 Zulu* (I don't know where you are so I'll use Zulu* time
) give you access to the most US players. Since the US is the largest online poker market, that's where the most bad players are.
Poker Stars and FullTilt are the biggest sites that accept US players, so they give you the most games since they have the most players. Poker Stars has the biggest tourney fields.
Welcome to the forum.
* - Zulu time = GMT uncorrected for Daylight Savings adjustments.