May Day.

The public holiday may be next Monday, but May Day is of course May 1st. This has been a time of celebration since forever. If you take the Summer solstice (June 21st give or take) as being the middle of Summer, then Summer starts at the beginning of May, and has traditionally been marked as such. By the way, the old saying "ne'er cast a clout 'til may be out" does of course refer to the may blossom and not the month. So May Day is the beginning of Summer proper, and that is why it is treated as a special day. The association with labour and trade unions only goes back a hundred or so years, and so is a relatively modern thing.