With the winter solstice being the shortest day of the year, of course we have the longest day of the year in the summer. This year [2021] that longest day is happening on Sunday. This year is seen as the true first day of summer (at least in the northern hemisphere).
A little history lesson: Litha (or Midsummer depending where you live) is all about growing crops, abundance, and, of course, welcoming the summer months. This is generally a fire celebration within the pagan community (bring on the bonfires!) Actually, in some areas, the Litha fires were done by setting a giant wheel on fire and rolling it down a hill into a body of water. This is symbolic of finding the balance between fire and water and it is also believed that by submerging the sun wheel into a body of water can keep drought away for the growing season.
There are some traditions that tell us that Litha is the time when the Oak King and the Holly King battle for power. The Oak King is said to rule between the Winter Solstice and the Summer Solstice then the Holly King takes back over from summer to winter. It all depends on which stories you like the most.
So how to celebrate:
Bonfires!
Get outside especially if you can get into a body of water
Enjoy fresh fruit (and fruit wine!)
Bring the outdoors inside
Meditate in the sunshine
Resources that I found helpful and have a lot of great information:
https://www.learnreligions.com/history-of-summer-solstice-holiday-litha-2562244