Travel blogs love the line "St Petersburg museums are free on the third Thursday of the month". There is no such city-wide day. Every museum has its own rule, its own eligible categories and its own way of issuing the free ticket — and in most cases free entry still means a ticket with a timed session, a limited quota and a document check at the door.
Kunstkamera: third Thursday, September to April only
The Museum of Anthropology and Ethnography (Universitetskaya Embankment 3) states that free admission for individual visitors falls on the third Thursday of the month and applies from September through April — there is no free Thursday in summer. The free ticket can be issued online or at the desk, is valid for entry within half an hour of the session start, and you approach the guard at the entrance rather than the usual control point. It does not cover the tower halls with the Great Gottorp Globe and the first astronomical observatory, which are guided-only. A regular adult ticket is 500 roubles, 300 for students and pensioners.
Russian Museum: the 18th, the 25th and a quota
Visitors under 18 and students enter free on the 18th of every month; museum staff and members of the museum workers' union on the 25th; Russian pensioners and people with second-degree disability on every third Thursday. If the date falls on a Tuesday, the museum's closing day, it moves to Wednesday. Children under seven are always free. The catch is published by the museum itself: on the free Thursday only 50 tickets are released per 30-minute session — 20 online and 30 at the desk on the day. Free tickets are issued only while seats on the session remain, and they do not cover guided tours, the Summer Palace of Peter I or the east and west pavilions of the Mikhailovsky Castle.

