Lughnasadh (pronounced lune-ah-sah) was a summer festival of the ancient Celts, celebrated around August 1. My understanding is that it is known that it was celebrated at either the full moon or the new moon closest to the midpoint between the summer solstice and the autumnal equinox. Since the midpoint is on August 6th, that means that this year, Lughnasadh falls on either July 25 (the new moon) or August 9 (the full moon). We celebrated the new moon this year; for the reason why, see the footnote below.
Legend has it that the god Lugh established the holiday in honor of his mother, Tailtiu. Traditionally, it was celebrated with trade festivals, fairs, and bonfires. In the modern Druid Revival tradition, Lughnasadh is the midpoint between Midsummer and the fall equinox, one of the eight primary holidays of the Sun Path.
For some reason, the company I work for, which builds dictation software, has not yet established Lughnasadh as a company holiday. So we decided to celebrate on Sunday, the 23rd, instead.