The song is about Midsummer which we call Jāņi and is celebrated on the night between 23rd and 24th June. Midsummer's Eve (23rd) is called Līgo diena. They are one of the most popular holydays in Latvia.
Bit of a necropost here, but I feel it's justified to raise attention for the grand larceny that occurred tonight. This man got robbed I tell you. ROBBED.