Go for it! Lots of people have early or half-year birthdays. A few years ago my daughter wanted to have a swimming party at a local pool so we moved it to May (her birthday is in January)! On her real birthday - we had a small celebration (we took my daughter and one friend to a fun restaurant, and then came home to open presents, etc.).
I remember a few parties of kids in her class that were held during the school year instead of summer. Kids (and parents) don't care -- everyone loves a party!
I wouldn't make a big deal of it. I would recommend giving your phone # for RSVP, though. It seems as if these days, some folks don't respond too readily (people get busy and forget). I just call the day before if i have not heard from some. I've also found that as my daughter gets older, she just wants to invite a few girlfriends over for a sleep-over birthday party. In that case -- I've called to see what date works out for everybody.