DD's birthday is in the holidays so we have always had a party in the holidays. Most come although probably not so good a turn out as in the school year ( which isn't an entirely bad thing).It's quite nice for them to meet up before they go back to school. The main problem is people saying they will come but forgetting by the time the party comes around.
DS is mid August and we either used to take 1 or 2 of his best friends on a special day (like Chessington) or have a party just before or after the hols - Sept worked best. Gave him something to look forward to after his birthday - invites went out on 1st day of term and they all turned up. Trying to have a party in the hols, even those who said they would come often forgot.
In the holidays you always risk the problem with people being away
DD's birthday is always in the Easter holidays. Have to admit that I always arrange her party t be the weekend before schools tarts, so sometimes 2 weeks before her actual birthday. This is simply because of the people being away issue.
In 2 weeks time we would be away on holiday. However one of DD's friends is having a holiday time party and DD can just make it. I knw the parents are concerned though as lots areaway and they are considering just turning this party - at home - to a tea party for a few available people and having a bigger party in September.