After contemplating this topic for a while, I feel the need to partially change my previous position.
Double EXP on weekends now seems like too much of a difference in relation to week days, which would, as some have stated, ruin parties from Monday to Friday.
That being said, I still think there should be some kind of EXP Event during weekends, preferably starting Friday night and ending at the last minute of Sunday.
Such event, though, should give us an experience bonus not higher than 50%, floating to as low as 25 or 30%. Besides, it should also depend on zeny donations from players to be activated.
I just don't appreciate the idea of having once a week or once a month floating rates that can be activated any time, for someone might simply donate on a Monday, for instance, and hinder most players from enjoying the increased experience rates when they actually have the time to play.