Ancient Latvian festivals are an equivalent of Celtic neopagan festivals in the Baltic culture of Latvia. Take into account that the article Latvian mythology, where most of this celebrations come from, is a disputed article in Wikipedia (August 10, 2010). But also take into account that some of the festivals, e.g. Jāņi, is a well-observed celebrations. 
Other festivities, as long as it follows the correct equivalent of the Gregorian calendar dates, are valid to be placed in the Holidays and observances section (e.g. the Incan Inti Raymi.)