Lammas (Loaf Mass) or Lughnasadh (Loo-nas-ah) is the third of the four fire festivals held at solstice and equinox midpoints. This one is at the Leo midpoint (15° Leo) twixt Summer solstice (Cancer) and Fall equinox (Libra).
Lugh is a sun deity. Nasad suggests harvest festivals, fairs, and games. It celebrates the midsummer harvest in the Northern Hemisphere (midwinter in the Southern Hemisphere). It is the third of the Celtic fire festivals when the celebration begins on the eve of the day.
Lammas ritualists, students, and the curious might be interested in precise timing for the cross-quarter moment when the Sun reaches the Zodiac degree 15 degrees of the sign Leo, the exact time between Summer Solstice and Fall Equinox. Popularly, Lughnasadh events begin on the evening of July 31 for an August 1 celebration. These times are for the actual cross-quarter moment.
Aug 7 2020 AD GC
01:06:17 AM GMT/UT
02:06:17 AM BST London
Aug 6 2020 AD GC
09:06:17 PM EDT Washington DC
08:06:17 PM CDT Chicago
07:06:17 PM MDT Albuquerque
06:06:17 PM PDT San Francisco
03:06:17 PM AHST Honolulu
For most of history, it was not possible to calculate the exact midpoint in time (above) that marks midsummer. Now it is possible to get that time. It is probable that in other eras, additional factors like a new or a full moon might be part of the decision as to when to light a festival fire. Do not think it wrong to celebrate, to recognize the cross-quarter, at another time.