Do we need to book more than 2 days ahead to travel from Munich to Zurich on December 20th. We can book at Munich train station on Dec. 18th. Would this be OK at the busy time of the year?
There are so many different trains and routes, that I feel sure you could find a connection with reservations at the station on the 18th. However, if you are already committed to that date, there are a number of connections with discounted, advance purchase online fares available (eg, the 12:33 EC, direct via Lindau in 4h11m at 49€/person). You can find these fare on the German Rail (Bahn) website.