March 25 is one of several days in the Latvian calendar designated as a day of remembrance for victims of Communist terror. The others are June 14 and the first Sunday in December. As of today, I have now been in the Baltic countries for all three of these days (The December one in 2009, and for the June 14 commemoration I was in Lithuania in 2005, where it is also a day of remembrance).
Why March 25? Because on this day in 1949, a several-day operation began to deport approximately 90,000 Balts to Siberia (more than had been deported in June of 1941), and the largest group of them – about 41,000 – were Latvians. The majority of the deportees were either women or children, grown men making up only 27% of the deportees.