Linda - you can take a soil sample at any time that the soil is not frozen.  Just make sure the soil is not packed hard or too wet when you send it to the lab.  Air drying overnight is a good method to get it to room temperature and reduce the moisture.

I've included the link below where you can purchase online an MSU soil sample kit or perhaps purchase one from the Oakland MSU Extension office.  If purchasing locally, I would give the office a call to confirm they have the test kits and the office hours.  The kit includes everything you will need for the soil sample and detailed instructions.  Results usually are emailed to you within 2 weeks.  We can view your results from this site so let us know if you have any questions.  Hope that helps.

https://homesoiltest.msu.edu/get-started