The festival has many names, all over Europe. Here in England the name is Lammas. It celebrates the first Harvest of the year, and is a Sabbat. What the altar is dressed with would depend on the country harvest.In the UK the first harvest is all the "table" vegetables. The second harvest,corn,wheat,etc. is at Mabon. The main ceremony of Lammas, at least in the UK, is to climb the nearest mountain (or big hill!). In some of the more remote villages they still have the wild goat,Mr Puck, and parade him around the village. But that ceremony has just about died out these days.
Celebrations (in the UK) start in the evening of July 31 until the evening of August 1st. Some groups spend all night on top of the mountain or hill!