Well, if you're expecting it on the seventh of every month, that's your first mistake. Some months have 30 days and some have 31, then you have February. It's a really really good thing to start keeping track of your periods. Do you do that? I've been keeping track of mine for years now, and it's a great thing. You can go back and see if your periods space out a bit more when the seasons change, you can determine if there are other patterns at all as well. A normal woman has her periods around 28 days apart. However, some women have their periods 21 days apart, and I used to be five to six weeks apart for ages. Just remember that your body goes on its own schedule. Some months you may be a day or two earlier or later than the last, and that's totally normal and okay.
Also, I just want to say, don't get on the hating your period bandwagon. It's really not that bad. If you have bad cramps or symptoms associated with your period, it's probably your body trying to send you some messages about your life in general, and it's a good tool to use to learn about yourself. If you hate your period, you will only compound any difficulties you have and end up making it worse. It's just a part of life, and if you look at it in a diffferent light you can think of it as a time to just pamper yourself for a couple days, or a good excuse to slow down and think about how the last month has gone for you. It's all about perspective.