The main themes are harvest and the Sun God Lugh (pronounced "Loo"). At Lammas he's slain and will be resurrected at Yule in the form of the new God (much like the Christian Jesus). Some people make a Corn King to represent Lugh.
Here are a few links with more info about Lammas aka Lughnasadh (also click on Lammas at the start of the post to read more about it):
http://www.thewhitegoddess.co.uk/the_wheel_of_the_year/lammas.asp
https://www.learnreligions.com/celebrating-lammas-or-lughnasadh-in-august-2562156
https://thepaganandthepen.wordpress.com/2010/07/31/lammas-corn-king/