This is a pun on 'mourning' which is the expression of deep sorrow for someone who has died which is related to the profession of Undertakers as they're a person whose business is preparing dead bodies for burial or cremation and making arrangements for funerals. 'Mouring' is also being used because it sounds exactly similar to 'morning' which is when you'd get up early
Undertakers have to get up early in the mourning .