This special event callsign is devoted flight in space on June, 16-19th, 1963 a spacecraft "Vostok", piloted by the first-ever woman-cosmonaut Valentinoj Tereshkovoj worked on air under callsign "Chaika".

Valentina Vladimirovna Tereshkova ( born 6 March 1937) - Soviet cosmonaut

Biography - http://www.qrz.com/callsign?callsign=R45VT 

The callsign is active on air every year on June 16 + 7 days (1 week).