When they are that little, personally I think a 'half birthday' thing in summer is just confusing. My son's birthday is the week BEFORE Christmas, so I know where you're coming from.
We have always had his party in December- I offered a summer half birthday once, and he told me he really LIKES having his birthday when it is and didn't want to change it!
Our last party was a Nightmare Before Christmas party (age 10) and since we already had the Christmas decorations up, we also put up Halloween stuff and had a Jack Skellington cake and all the boys wore their Halloween costumes and played games like 'wrap the mummy'. It was super fun.
Just call up a few moms and figure out a good date that works for you. Remember, no matter when you have the party or what you do, there will always be a few kids who can't make it. I would just do it the weekend after Christmas or you could try and squeeze it in the week before- but after will likely be easier for most people.
Have fun!