John is talking about their group's habit of hanging out at the cemetery to drink. He is saying that Masterson's Tomb, the specific place where they like to meet, is a great place to be in the summer, when it is warm. The cemetery offers him and his friends a place to gather where adults will not bother them, providing a sort of escape from authoritarian hassles in their lives. In the winter, however, it is not at all pleasant to hang out at Masterson's tomb because it is so cold. In December, reality hits, and what was an idyllic place to withdraw from the cares of life in the summer becomes a frigid nightmare in the winter (Chapter 10).
The part of the cemetery where John and his friends like to gather to drink is called Masterson's Tomb because
"that's where all the famous Mastersons are buried...it's a fantastic place because they have acres and acres all for their own tomb, and it's fenced in with a private road which they only open up when one of the Mastersons dies. But there is a hole in the fence at one place in the woods, and that's where all the kids go through...The tomb is a great big marble building that's set in the side of a hill so only the fancy front sticks out...it's all chained up, so (the kids) climb up the side of the hill and get on top by these two glass domes that let you peek down inside...(even though) you can't actually see anything" (Chapter 7).
John makes the statement about Masterson's tomb being "a realist's nightmare in December" around the Christmas holidays. It is the time of year when it is freezing at the cemetery, so John is glad that this year, they have Mr. Pignatti's house in which to hang out instead (Chapter 10).
They say it is an escapist's dream in the summer but a realist's nightmare in the decmeber because its very beautiful in the summer but horrid in the winter.
this qoute means that in the summer the graveyard is alive with plants growing. In the winter everything dies and not to mention the dead bodies. hope i helped