What does father realize it is the anniversary of? Describe what the family does to celeberate.
You do not specify which part of the book you are talking about, but I think you are probably referring to the part where the father realizes that the next day will be the anniversary of their escape from the shipwreck. He declares that they should have a day of thanksgiving.
For details of the celebration, see Chapter 9. They started with the boys setting off an explosion in celebration, waking their parents. They spent the morning quietly. They thought and talked and prayed about the the year that had gone by. Then they had a whole bunch of competitions for the boys in the afternoon.