It may be the 3rd May now, but I still have a little bit of research that I'm sharing here. As well as looking into Celtic origin Festivals on the 1st May, I've also found some Ancient Roman celebrations for May 1st.
The public were called to the Curia Calabra by the Pontifex minor. The Curia Calabra was a dedicated religious location, a temple used for the new moon ritual observation. The Pontifex minor (one of the College of Priests and not the High Priest who was the Pontifex Maximus) and the Rex Sacrorum ( a nominated priest of the patrician class) offered a res divina, a religious sacrifice to Juno. Janus was also invoked on the first day of May.
Maia’s Sacrifice - 1st May.
A priest of Vulcan sacrificed a pregnant sow to Maia, the usual offering to an earth goddess. This event seems to have been linked to the Bona Dea dedications.
Bona Dea temple dedications - 1st of May
Damia (Bona Dea) the “good goddess”, was the Roman goddess of prophecy, chastity, fertility, healing, the protection of individuals, the Roman state and fauna. Honour was given in Bona Dea ritual sacrifices and they were women-only services. The priestesses of Bona Dea (Damniatrix) sacrificed a sow (a cow that had produced offspring and was named the Damium). The Bona Dea temple on the Aventine was decorated with snakes and herbs used for healing. Women carried honey pots into the temple, filled with ‘milk’- offerings of wine and they decorated statue of the goddess with vine leaves.
The festival of the Laribius/ Lares Praestites gave honour to the public lares as protectors of the city of Rome, and was held near the temple of Vesta. The Lares were also protectors of a personal home. The household occupants worshipped their own lares choice at the lararium, a dedicated area or niche which held the sacred image. Lares worship was expected on the kalends (1st day of the month); the nones (5th or 7th depending on how many days in the month); and the ides (15th or 13th depending on how many days in the month). Though it was recorded that many people gave offerings to their lares every day.
Meanwhile the Floralia festival, which began on the 28th April, continued…