How do I lie to my parents about going away for 4 days?

Basically, me and my girlfriend are going to Kent for a romantic long weekend/Harry Potter Convention (Diacon). I'm 17 and my girlfriend lent me �200 to cover the costs as I didn't have it, on the basis that I will pay her back. My parents originally agreed (I lied and told them it cost �100), but recently they have changed their minds about it. I'm definately going as the tickets to Diacon are non-refundable and I have dreamed about it since I was a little boy. The only trouble is, both of their birthdays are during the time I will be away, so I have no idea how to lie to them about it which will excuse me from missing them etc. Please help, this is really important to me, my girlfriend might break up with me if I don't go.