Well, not just Latvian tourist authorities promote the country as "the
singing nation", so does every Latvian :) Yes, the people very much belief
so that Latvia is the singing nation. Often Baltic people from all 3 states -
Estonia, Latvia, Lithuania do not know that they all 3 are calling themselves
that way :) So it's almost a regional anecdote, but anyways - it's apparently
better to say that Baltic's are the singing nations :)

One more thing - the song festival is not called "Dainas festival".
"Dainas" is "folk song/rhyme", a folk poetry without [known]
author. The correct title is "Dziesmu sve-tki" (Song festival), but it's
often called "Dziesmu un Deju sve-tki" (Song and Dance festival), since,
though Song festival and Dance festival are actually separate festivals, they
usually happen at the same time every 4 years, so they are usually perceived as
one festival.