How can I advise my friend about periods?

So my friend, a bit older than me, has just started her period. I don't really know how to explain to her that her cycle almost certainly won't be regular, when I asked her to come for a sleepover she said she'd be on her period then, keep in mind, as far as I know she's going by the calendar on her phone. I know, that, if it doesn't follow the calendar, she'll worry. I dunno how to explain this to her? Tips and suggestions, also does anyone have funny, embarrassing or down right gross period stories I can read to her?