Definitely stand your ground in the christmas/birthdays. However your children WILL be made to feel very guilty for continuing to celebrate them, so like I said, make sure you reach them that they have their own minds and don't need to be bossed around or made to feel bad by doing things differently than others.
your wife wants you to go to the memorial first because there's lots of visitors at the memorial and it's not so unusual for new people to be there. You won't make as much of a scene as going to a regular meeting.