1 August is Lammas day, the traditional first day of the harvest. Different communities have their own celebrations related to this day. In my family we celebrate my Uncle's birthday, which is one reason why I mark it.
The name Lammas comes from Anglo-Saxon, but it's also considered part of the Celtic calender, under the name Lughnasadh in this sequence:
Imbolc (1 Feb) (Start of Spring)
Beltane (1 May) (Start of Summer) (also Gemma's birthday)
Lughnasadh(1 Aug) (Start of Harvest)
Samhain (1 Nov) (End of Harvest; Start of Winter)
So far as I can tell, no apostrophe is needed for Lammas day (it isn't the day of the Lamma).
 Meaning "Lugh's Wedding" - Lugh was the a Celtic God who probably didn't give his name to London.
 Traditionally marked by Ewes lactating. Sorry if this is too yucky an indicator for you.
 The herds were moved up to the summer pastures on Beltane (or, more likely, the day after if it was celebrated properly).