As you know you can't guarantee good weather on a particular day each year. On my eldest sons 3rd birthday (in the middle of October) it was shorts and t-shirts weather and we held the party in the garden. In fact it was warmer than it is today! We got married in February, so we knew that the weather wouldn't be brilliant, but that's when we wanted to get married. Just go with when suits you best, and blow the weather, if it's nice it's nice, if it's not, well, I promise, it won't spoil your day.