How long does birth control take to work?

Cassie
I'm going to the doctor to get birth control in a few days - and on the 26th this month I'm going away, and my period is due on the 20th. I really want my period to come on time (I'm naturally irregular) so it'll be over by the time I go on holiday. How long does the pill take to take effect in regulating periods? 
Thanks!!