The event filter takes a single argument which must be a quoted string. It is true if the argument string is a substring of the Event field of the game.
    event "Open"
An event filter may also be included as a CQL parameter, in addition to be used as an ordinary filter. For example,
    cql (input i.pgn event "Open")
    countsquares 4 10 q

event "1.p"
not event "@"

The earliest correct (tested by the not event "@" above, using the conventions of the Heijden database) 1st prize study is from 1881.

The event filter is used in the sample file gameinfo.cql .