when do you shave a premature muslim baby's hair?

Just wrap its head in a towel and don't worry about it.

Premature Baby Hair

A baby's hair is to be cut on the seventh day, regardless of whether it's premature or born on time. However, if the bub is unwell, the obvious thing to do is wait until it's well enough for that sort of thing. Some of the scholars reckon if you don't cut it on the seventh, then do it on the next seventh (the fourteenth, or the twenty-first, etc). However, I don't know what the proof is for that ruling.

To the poster who wrote to "wrap its head in a towel". Dude, this is a baby you're talking about -- really, do you have to act that low?

It is sunnah to shave the head of baby boys on the 7th day. It is not sunnah to shave the baby girl's head.

http://islamqa.com/en/ref/334/

http://islamqa.com/en/ref/14248/

http://islamqa.com/en/ref/7889/

take the baby to the church,