I've lately been using Google Calendar to keep track of plans. I don't have that many plans, and not everything goes on there (like last-minute stuff), but I generally try to put things on there that I know about, as it helps me remember them and not double book and whatnot. In the past I've been kind of bad about keeping track of things, but this seems to be working for me. What probably makes it most useful is that I use Google for my email, so all I have to do to get to the calendar is click a link at the top of the mail page.
So, the default behavior is to have the week start on Sunday, which is not so useful for me for looking at the coming week, since I like to know about my Saturday and Sunday plans, and having to go to the next week to see what's going on Sunday is not ideal. So just now I realized that there should be a way to change this, and indeed there is. You can have the week start on Saturday, Sunday, or Monday, so I set it to Monday.
Of course, this makes me wonder why you can't arbitrarily set any day to start the week, since I assume that it's just as easy to program the week to start on Thursday, say, as Monday. I guess it's probably a UI thing. Pretty much everyone will want the week to start on Saturday, Sunday, or Monday (the only situation I can think of where someone might want the week to start on a different day is if they work a different schedule, with their weekend on a Wednesday/Thursday, for example). So if you give these people more choices, some might get confused, or at least start to worry that maybe they're doing something wrong by picking Monday as the start of the week. I'd be curious to hear other people's thoughts on this.