Why would a union go on strike in the winter?

I saw people picketing outside of a hotel this morning, it had to have been about 10 degrees.

Whatever crappy hotel job they have; it is still likely better than marching outside in the cold for free.
But even if it isn't, shouldn't they wait until April, when they can picket in better weather?

Apparently the conditions inside that hotel are worse than the freezing cold outside ;)

They have to go on strike when their contract is up, no matter the time of year. They can't choose the time of year.
i'm sure management would love it if people like you would join the union. union members don't choose the time of year that they picket. whenever the contract comes due is when they start negotiations. if those break down and management won't budge, its time to strike.