From the Clerk of Oxford:
August 1st is Lammas Day, the earliest Anglo-Saxon festival of the harvest - a day of first-fruit offerings, on which loaves of bread made from the first corn were blessed. The word comes from the Old English hlaf, 'loaf' + mæsse, 'mass'. And if you want to see the word hlafmæssedæg in the wild, as it were, there's an Anglo-Saxon charm for the protection of grain that goes like this:
So this is what you should do to protect your harvested corn from mice and other pests:
[...] lange sticcan feðerecgede 7 writ on ægðerne sticcan[...] ælcere ecge an pater noster oð ende 7 lege þone [...]an þam berene on þa flore 7 þone oðerne on [...] ofer þam oðrum sticcan. þæt þær si rode tacen on 7 nim of ðam gehalgedan hlafe þe man halgie on hlafmæssedæg feower snæda 7 gecryme on þa feower hyrna þæs berenes. þis is þeo bletsung þærto. Vt surices garbas non noceant has preces super garbas dicis et non dicto eos suspendis hierosolimam ciuitate. ubi surices nec habitent nec habent potestam. nec grana colligent. nec triticum congaudent. þis is seo oðer bletsung. Domine deus omnipotens qui fecisti celum et terram. tu benedicis fructum istum in nomine patris et spiritus sancti. amen. 7 Pater noster.
[Take two] long pieces of four-edged wood, and on each piece write a Pater noster, on each side down to the end. Lay one on the floor of the barn, and lay the other across it, so that they form the sign of the cross. And take four pieces of the hallowed bread which is blessed on Lammas day, and crumble them at the four corners of the barn. This is the blessing [you should say] for that: "So that mice do not harm these sheaves, say prayers over the sheaves and do not cease from saying them. City of Jerusalem [?], where mice do not live they cannot have power, and cannot gather the grain, nor rejoice with the harvest." This is the second blessing: Lord God Almighty, who made heaven and earth, bless these fruits in the name of the Father and the Holy Spirit. Amen. And [then say] a Pater Noster."
Lots more at the link.
The song at the top refers not to the Anglo-Saxon festival, although that's where the name comes from, but to the annual fair in Ballycastle, County Antrim which dates back to the 17th century. The Ballycastle fair, though, takes place at the end of August and not on the first.
Chambers's Book of Days has some Lammas Day traditions, too.
It was once customary in England, in contravention of the proverb, that a cat in mittens catches no mice, to give money to servants on Lammas-day, to buy gloves; hence the term Glove-Silver. It is mentioned among the ancient customs of the abbey of St. Edmund's, in which the clerk of the cellarer had 2d.; the cellarer's squire, 11d.; the granger, 11d.; and the cowherd a penny. Anciently, too, it was customary for every family to give annually to the pope on this day one penny, which was thence called Denarius Sancti Petri, or Peter's Penny.'—Hampson's Medii AEvi Kalendarium.