Question

why was M. Hamel in party wear on that day

Answer 1

Because that was the last day for him at that school and from the next day a new teacher would come. However he wore that dresses at the special occasion and that day was special for him.

Answer 2

Answer:

   This day was a bitter yet important day for him because it would be his Last french classes in  the city of Alsace. French along with Hamel had to vacate from Alsace. As it was the final privilege of his teaching career at Alsace he came in party wear.