irregular period...help!?!

i have never had a problem with my periods until about 6 months ago. i can't seem to accurately predict my period anymore. for example, i was supposed to have my period this past saturday and it still hasn't come. i know it's only been a few extra days, but this happens every month now, and it can go up to a week late. how can i be prepared for it if i never know when it's going to come?