system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Since the shortest day of the year in the northern hemisphere is December 21, how come it's either January or February the coldest months of the year?

The same goes for summer. 
Bonus : If it's some kind of "momentum" why does it take so long?
gpt
You're right about the "momentum" thing, sort of.  The Earth is a sort of warmth battery -- you know how on a hot summer day, the ground will stay warm to the touch even after the sun goes down?  This is the same sort of thing, but on a much larger scale.  It takes about a month for the Earth's surface temperatures to reflect what the sun's been providing.  It's a big thermal mass, and so it takes a long time to change.