If there's a great time to visit the Netherlands, it will have to be during the Christmas season. The Christmas markets in the Netherlands aren't as awesome as the ones in Germany but they're still amazing. Both Amsterdam and Rotterdam have okay Christmas markets, but the nicest ones are easily Maastricht and Valkenburg.
Personally, I would head to the Christmas markets along the dutch/German border. Christmas markets in Netherlands aren't really anything like you'll find in Germany and they're mostly cheap imitations of the German ones. If you can, head to the Christmas market in Dusseldorf (it's just over 2 hours by train from Amsterdam).
Cologne Christmas markets Cologne is easily the best and most popular of Christmas market in Germany. I had the opportunity to visit Cologne as a day trip from Amsterdam and you can easily get there by train as it only takes less than 3 hours. The beautiful Old Market in front of the Town Hall and the Market of Fairytales on the Rudolfplatz are the most popular markets.
Dutch border christmas markets The Christmas market in Oberhausen is the largest in Germany and it's only right across the Dutch border (about 100 miles from Amsterdam). Another great christmas market in Germany close to the dutch border is the one in the picturesque village of Monschau, though this one is closer from the dutch city of Eindhoven.