German Christmas Markets normally start after the "Sunday of the Dead" has passed or shortly before the 1st of advent. The biggest ones will go on all through december 24th. On the 24th - called Heiligabend - most will close around noon as many people need to prepare for Christmas Eve. As it is not custom to prepare a tree in early december as maybe in the US, most Germans will put up a tree on the 24th. Up till now my family always had real trees (this year it will be one from our own garden - which means spider invasion shortly after bringing the tree inside-----yeah! I'm so not looking forward to that...), so this has practical reasons as well.
Obviously going to a Christmas market in broad daylight isn't as much fun as going at night.
