This could be attributed to the seasons of the year - how much people are outside and away from their work, how much nighttime there is - except for peculiarities like the January (New Year's hangovers?) spike and the high number in November. But if this is the case, an inverse patters should hold true for the southern hemisphere. But instead the pattern is essentially the same.
Furthermore, while there is the expected increased number of daylight disc sightings in the summer months, all three major Hynek categories (Daylight Discs, Nocturnal Lights, and Close Encounters) follow the same pattern:
And most astonishingly, the southern hemisphere again follows the same pattern, except the increased number of daylight discs in June-August is suppressed, and the percentage correspondingly greater in December-March (southern hemisphere summer):
So, again, the Cosmic Joker - an unaccountable pattern, not tied to any obvious natural or human cycle, but keyed to the human calendar year….