Depends on what I'm playing.
I play predominantly low stakes SNGs ($5,6,10,11) and prefer to play on weekdays 12:00->17:00. Almost never on weekends.
The play in the micro SNG's can be so bad on weekends and my biggest leak is a tendancy to get pissed off. I prefer a SNG with at least a few good players on the table, as opposed to mostly shitty ones. I run far better on a marginally better table. Call me an idiot but my stats. show this to be true for me.. by a longshot.

For 6-MaxNL, weekends..... anytime at all.

MTT's, weekdays again (for the buyin level I play at, $26).