It is clear in Act I scene 4 that Hamlet strongly disapproves of the feasting and drinking that is going on to celebrate his mother's marriage to his uncle. Remember that this feasting and celebrating must seriously annoy Hamlet, who is stricken by grief and still trying to get over being disinherited and also trying to accept that his mother, who was apparently so in love with his father, has chosen to marry his uncle. He is therefore not going to look kindly upon the king's feasting and drinking at a time when he feels everybody should still be mourning the former king, his father. Note what he says to Horatio about this custom of drinking:
It is a custom
More honoured in the breach than the observance.
Hamlet thus feels that although the carousing and drinking is a Danish custom, given the circumstances, it would be more appropriate not to follow it and to mark the occasion by breaking that custom given the recent death of the fomer king.