Job 3:1 "...Job...began to call down evil upon his day (yome)"
Job 3:3 "Let the day (yome) perish on which I came to be born"
'yome' is here referring to Job's birthday.
Job 1:4 "And his sons went and held a banquet at the house of each one on his own day (yome)"
On Job 1:4 Barnes commentary adds:
Perhaps it refers only to their birthdays; see Job_3:1, where the word “day” is used to denote a birthday. In early times the birthday was observed with great solemnity and rejoicing.
Clarke commentary adds:
Feasted in their houses, every one his day - It is likely that a birthday festival is here intended. When the birthday of one arrived, he invited his brothers and sisters to feast with him; and each observed the same custom.
This isn't concrete, but it's certainly a better argument than the WT's "the only examples of birthdays are bad", which is incorrect because it fails to mention Noahs' birthday in Gen 8:13 where something GOOD happened - the waters of the flood had drained away.