Unlucky - the time of the month is set to strike the very day you jet off. Not exactly great for swimming, diving and parading around in the tiniest of minis, to say nothing of the decidedly unglamorous side-effects: stomach ache, migraines, fatigue, breast pain, bloating, irritability and all the rest. AND sex on your period isn't exactly what you had in mind. Being on your period on holiday is a nightmare with a capital N.
Delay your period
Don't worry: it's not the end of the world. If you want to, you can either delay or miss your next period. Plan in advance and speak to your doctor: he'll advise you what to do depending on what Pill you're taking.
If you are struck when you're away, don't forget to change your sanitary towels or tempons regularly (every 4 hours). It's not always easy when you're out and about or at the beach, but changing regularly will reduce the risk of infection. Remember tampons still allow you to swim!