Thanks for everyone's suggestion!
Unfortunately, Jan seems to be the only time we could visit....since we have been to so many museums in our previous trips to Europe, we are not sure if we want to fill our full of museum visits (but of course, we will still visit some of the famous/main ones). Because we haven't visited Germany, so this time the main focus will be Germany :)
We visited Amsterdam in late Nov before (it was already cold in late Nov), but only stayed for 3 days (2.5 days actually after taking out the travel time) and thought it wasn't enough so we wanted to go back. Would it be better to visit Amsterdam during the flower blooming season instead (skip Amsterdam this time)? Because we are flying in to Amsterdam, so we thought we could spend a couple of days there? or should we fly to Germany right after arriving Amsterdam?
thanks!