system
Given the following post, respond as if you're commenting on Reddit:
human
When a seed requires 6 weeks of cold weather to germinate, what makes it take so long -- i.e., what happens at week 6 that didn't happen before, or how does the seed "know"?

It's not necessarily about freezing, either, as there are seeds that you can germinate in a fridge (staying above freezing).
gpt
Plants (and seeds) sense temperature in a number of ways, though it is not precisely known how seeds 'count' how many days/hours they've been at cold temperatures. However, as others have pointed out, t is known that plant hormones such as jasmonic acid, abscisic acid and gibberellins all play an important role in either inducing or repressing dormancy and germination.  

Some of the ways plants sense temperature changes may give us some clues as to how they might 'count' how long they've been at a certain temperature. For instance, plants actively adjust the ratio of saturated to unsaturated fatty acids in their membranes to maintain fluidity and functionality of said membrane in response to decreasing (or increasing) temperatures. By decreasing the ratio of saturated to unsaturated fatty acids (i.e. adding more unsaturated fatty acids) the membrane will remain fluid at lower temperatures.

Now, if you think of a seed as a bank and each structure (organelles, membranes, etc) within the seed as a separate bank account, with transactions between accounts being represented by biochemical pathways and money being the separate chemicals and molecules involved in these pathways, it could be that by draining one 'account' continually over time (like the account that produces the precursor to unsaturated fatty acids) in order to increase the amount of 'money' allocated to another 'account' (the membrane), it could be that the plants eventually realize, "Oh shit, its been cold a long time, maybe we should germinate the next chance we get because this really sucks."

On the note of hormones: they, like fatty acids, have to come from somewhere; often they have to go somewhere as well. Source and sink. So, if the enzyme that produces the hormones from whatever precursor molecules and the enzymes that move these hormones from the source to the sink are both more active at lower temperatures than the enzyme that senses this hormone at the sink, it's likely that the hormone will build up within that sink (a tissue, organ, etc). Excesses in certain hormones at a given location past a given threshold can lead to genetic reprogramming; it can turn genes on and off. So, if the sink enzymes are not active, and the concentration of the given hormone continues to increase over time, it could then activate genes that cause germination. On the other hand, if the temperature isn't low enough (and therefore the sink enzymes can effectively process this hormone in some way) or the duration isn't long enough (and therefore the concentration of the hormone doesn't reach a particular 'gene activation threshold') the genes may never turn on and lead to germination. 

tl;dr No one really knows exactly how seeds do it, but hormones and membrane fluidity could be a potential signal.