Summer Solstice is often referred to as “midsummer”, and had differing traditions surrounding it in ancient pagan times. Among the Greeks, there were some calendars which pivoted around the summer solstice. In Athens, their calendar went from summer solstice to summer solstice and in Delphi the new year started with the first new moon after the summer solstice. Other calendars began around the winter solstice or other dates. Their calendars were mostly lunar oriented with some solar events thrown in, and often different regions had multiple calendars. In Athens this was celebrated with the holiday of Kronia, which was a harvest festival. It was dedicated to Kronos, father of Zeus. There is also evidence to suggest that in the Delphic calendar (and possibly others), the summer solstice was celebrated in honor of Apollo.
In modern times, some Hellenists have chosen to go the Athenian route and celebrate their own version of Kronia around this time of the year. Others have chosen to celebrate Prometheia, a modern festival which honors Prometheus. Still others have gone a slightly different route along with other pagans who also mark this as a harvest festival but have taken on customs which are more Celtic in origin such as the bonfires, purification and blessing of crops, etc. As a result, it makes sense to consider honoring Apollo during this time of year–and there certainly appears to be ancient precedence for it. It may even be interesting to note that Apollo’s connection with harvests came first and the sun later in association with such. There certainly would be nothing wrong with combining various elements of these festivals and honoring all three at once, or simply honoring the day with a libation to Apollo and other related deities as various groups in Greece have done.
Given how many calendars and traditions there were around this time of year–and are–you honestly can’t go wrong no matter what you choose. I personally honor Apollo on this day, especially due to his connections with this time of year at Delphi and his associations with agriculture and purification.