Hi Mummyamy, I don't think there is a right or wrong answer. All the advice I can give is from my own experience.
I am enjoying the diet and after a few weeks am finding it easy to stick to. It's quite a dramatic change and can be hard the first couple of weeks so you really need to want it.
I don't think starting the diet and then having a meal will cause you any harm, but it will be a huge effort for you to stop eating again afterwards. The hunger stays with you for a few days and in effect if you start on Monday and then eat at the weekend, you will have to start over again with hunger! If I were you, I would either wait until after the wedding.
I did have a birthday party to go to which involved going to Jamie's Italian for dinner two weeks into the diet. I agonised over the menu for ages and decided in the end that I didn't want to eat and stick to the diet so I took a bar to the restaurant and explained I was on a special diet and had a couple of coffees and a bar instead. Is this something you could consider?
Good luck and let us know how you get on!
Fiona