@nikitonsky It's because it was prophesied that the god Set could not be killed by anyone who was born on any day of the year, or in the night. So Thoth the wise went to Khonsu the moon god, gambled with him, and won some of his light, from which he made five new days that are not part of the year. (This is why the moon's light wanes.) The god Horus was born on one of the five new days, and destroyed Set.

Hope this helps!