"Mother" and "father' are
people and the days chosen to honor them are called their days. So
the possessive forms of "mother and "father" respectively are used
to name their days, i.e.. "Mother's Day" and Father's Day".
"Labor Day" is a day that
has been chosen to honor all working people. "Labor" refers to the
work done as well as the workers collectively. we don't normally use
an apostrophe 's' after the word "labor" to indicate possession.
Thus "Labor Day" is used.
"Laborer's Day" does not
sound so neat.
As for " Christmas", the
word means " the mass or festival of Christ".
"Christmas" is not a person, so we
can't put an apostrophe 's'
after it. "Christmas Day' means "the day of the festival of Christ".