I'm no wildlife expert but when a nocturnal animal is out during the day it could be ill, perhaps rabid. If it is out and about over a few days or it seems aggressive, animal control should be called.
I was curious, so I looked into it before posting. As squirrelitude says, it's both remarkably unlikely for opossums to contract rabies and not unusual for them to be out and about during the day, especially in winter.
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