When do Michigan landlords have to turn on the heat for the season?

I am not aware of any law that specifies a date for heat to start. In Michigan, landlords have a duty to keep premises "habitable." Habitable means that water, sewer, light, air, heat in the winter and so forth are provided. In order for the law to get into this level of detail, I think you'd have to be in a court and have convinced the judge that it was so cold that the home was uninhabitable because the heat was not on. This might be fairly difficult so far this year, since many of us have not turned our heat on yet. :) Though, I'm sure that many people have.

There may be requirements for government-subsidized housing, but I am not aware of any requirements for unsubsidized housing except that you are entitled to a "habitable" apartment. If it is getting a little chilly in your apartment, put a complaint in writing and send it to the landlord. This sets the stage if at some point in the future it is consistently cold and you still do not have heat - you may have grounds for withholding your rent.