First contractions?

[MissBax]

Not long off my due date now, and this may sound a stupid question - but what do your first contractions actually feel like? I've been having period-y cramps on and off the last few weeks anyway, so will the first contraction feel just the same (and then increase in strength and time), or will I "know" with the first one? How debilitating are they? If I'm out in public and get the first contractions am I going to be able to drive home etc?
The main reason I ask is I've been invited to a wedding a week before due date - I have already explained I may not be there as it's so close, but if I did go and suddenly labour began am I going to be able to excuse myself and drive home (about 30 miles)?
Cheers

My waters broke first (not textbook but not uncommon either). Contractions started about 20 mins later as a dull but uncomfortable ache at the bottom of my back. I could've driven for the first 3 hours or so but after that I'm not sure.

Tbh, I'd go to the wedding! I was at the cinema during the afternoon and out for dinner in the evening when I was 39+5. Just take your hospital bag and notes with you.