Valga (Estonia) and Valka (Latvia) were for centuries a single city in Livonia that went by the German name of Walk. However, when Latvia and Estonia won their independence in 1918, both felt the city should belong to them, and it ended up being divided into two separate cities straddling the border.
EU and Schengen Zone accession have seen border checks disappear and residents can walk from one country to another to shop, visit and even to order to a pizza on one side to collect on the other.
The next six days will be filled with fun events including a 'Borderless Party' on June 7, a Fair on June 8, a Walga-Valka Race on June 9 and a Big Children's Party on June 11.
For more info about the festival see: https://www.valga.ee/valga-valka-kaksiklinna-festival and there's even an English-language rundown of the events here.
You can read some reflections on Valga-Valka – or should that be Valka-Valga? – in this recent feature.