Well, an asteroid hits the moon, knocking it into a closer orbit which in turn messes with the Earth's tides, climate and tectonics (lots of volcanic eruptions which also messes with the climate). A family in Pennsylvania (in the first book) and in New York City (in the second) learn to cope with the new post-apocalyptic world. This generally consists of less food, loss of services, uncertainty about family members not immediately local etc. The 'event' occurs in May and people spend a lot of the first summer waiting patiently for the government to put things right... and then in a panic when that doesn't happen. The first winter starts early and is brutal with many many people dying of starvation and disease.
The first two books tell essentially the same story from two different viewpoints through the first year, with the first being a better story and having more sympathetic characters. Book three brings the survivors of the two families together through the second year. I did not like the third book nearly as much, the main characters lost all the sympathy that you built up in the first two books by acting incredibly selfishly and... well, this isn't a spoiler post so I'll leave it at that.
All three books are a quick read and of course, required reading for post-apocalyptic fans.