Neither one! Pick Thursday! For me starting on Thursday makes my period start on Sunday or Monday afternoon. This makes it done by Thursday or Friday, and I'll never have it on a weekend.

Just another option of course lol. Saturday and Sunday would make my period start on Thursdays and on the weekend I'd be right on the heaviest part. Its hard to enjoy weekends with cramps and heavy bleeding! I'd much rather put up with it during the week. This of course may be different for you.
This may not help but it might give you a different perspective.
