All I can say, from my entire life of having 3 older sisters, and years of having mostly only female friends, the only "right" thing a man can do for one (two if you're me ;

week(s) a month, is fucking hide.
There is no right thing, only a large number of wrong things. Hiding only works if you aren't obvious about it, you need to emerge on occasion to say "Hi," nothing more. Don't say "How's it going?" "How do you feel?" or any kind of question. If she's angry go back to hiding. If she's sad, give her a hug and an "I love you." Any other random mood you can try acting normal (although a lot more cautious) and pretend it's just any normal day.
Rules for PMS, and women in general:
1 - Never say the word PMS, or imply that she has it, ever.
2 - Never provoke anger directed at you, ever. Say "I'm sorry" and sound like you mean it.
3 - If she's mad at someone else, she's not mad at you. Agree that the person is a bitch, asshole, or whatever. She'll calm down after blowing off steam about how much she hates them, hopefully.
4 - If she's sad, comfort her because you love her, even though you are oblivious to her cycle

5 - Think about what you are going to say very, very carefully before you say anything at all. If it can be taken the wrong way, reword it and think about it some more, or just smile and say nothing at all.
6 - If she asks what you're thinking about, say something like you were just thinking about how lucky you are to have her, and how much you love her.
7 - Some women load up on perfume for their period, because there are odors, and she thinks that it's worse that it actually is. Notice it and tell her how nice she smells. The same goes for a new outfit, hairstyle, or any number of similar things.
8 - Chocolate and ice cream are awesome. Go out of your way to let her see you enjoying some, tell her it's delicious and offer her some. Know what types she likes, and make sure that's what you're eating.