My ds is end of Jan, so crapola weather, but no holiday issues (a month after is a long time to a kiddo!). Our neighbor's kid is Dec. 23 and no one EVER has time for a b'day party. They do one in the summer, and nothing for friends at the time of her b'day. They have a family celebration, which she doesn't get twice, but she only has one party for her friends (ie: when you go to her party, that's when you bring the gift, kwim?).
I think it's really up to you to decide - I like that they have the party when more people are around to participate, and that it's less of a stress on the parents of the kids invited, but I would NOT have two parties, it just seems too much for any kid. I voted I wouldn't do it because in general I think it's hugely kowtowing to the western idea/ego of self as so important, the whole b'day thing (and I buy into it myself, I'm not at all judging here), but to put it off 6 mos just for convienence seems too much to me.
That said, my best best best friend ever in the whole wide world has had at least three parties for her dd every single year for the past decade. Parties that included overnights in Manhattan with plays and dinner out at funky restaurants, Japanese restaurant and sleepover for FOURTEEN children, one year a Dora themed party (obviously they were small) for thirty three children. No kidding. So obviously we have grossly divergent ideas of what's appropriate, and she's still my best bud.
Don't even get me started on goodie bags!