Sunday is the best day as there is the most players playing. Unfortunately Sunday does not like me it is day I loose the most money. Monday is my profitable day so probably my favourite. It is not just Pokerstars I have won the ACR Cardschat freeroll the most on Mondays.
As others have said, games do tend to be softer in the weekends for fairly obvious reasons. Less people working = more recreational players having time to play poker. This is on an "everything else equal" basis though. 2NL cash games played at the worst time will still be softer than 25NL cash games played at the best time. But if you are trying to move up and take shots, it makes a whole lot of sense to do it, when games are best.