The Christmas markets are in general open from the Friday before 1st Advent Sunday (= fourth Sunday before December 25th, the Sunday from November 27th to December 3rd inclusive) until noon on Christmas Eve December 24th.

They are open from about 2pm on week days and from 10am on the weekend (Saturday and Sunday) until at least 9pm. 

List of all Christmas markets (in the same locations every year):  http://tinyurl.com/munich-christmas-m...

See here for a map of Christmas markets in Munich

Recommended Christmas markets:

Medieval Christmas Market

Everything is medieval, a lot of handmade articles like leather bags, bows+arrows, everybody dressed in medieval clothes, even the prices are in Gulden, 1 Gulden = 1 Euro

Brienner Str. 6-10 (near subway station Odeonsplatz, 10 minute walk from Marienplatz)

http://www.mittelaltermarkt-muenchen.de

Schwabing Christmas Market (artistic, a bit mad)

On the square above subway station Münchner Freiheit (subway line U3, U6)

http://www.schwabingerweihnachtsmarkt.de

Haidhausener Christmas Market at Weissenburger Platz 

Traditional, excellent crêpe with sugar and cinnamon

Near the S-Bahn stop "Rosenheimer Platz" (all S-Bahn lines stop there)

It's in a residential area and visited by families with children and it's just a 5 minute S-Bahn ride from Marienplatz.