It is custom to cut branches from cherry or apple trees on the 4th of December and put them in a vase. The branches are supposed to be in full bloom for Christmas. The lore says that if the blossoms open on Christmas eve the year ahead will be a very good and happy one.
An Austrian phytogenetic website suggests to hit the branches with a hammer (the bottom of the branches) and put them in warm water, this will enable them to bloom for Christmas. I am not keen on the hammer method and our branches usually bloom anyway – not always on Christmas though.