citsi gismu

x1 is a season/is seasonal [cyclical interval], defined by interval/property x2, of year(s) x3.

Also anniversary (= citsydei, na'arcitsydei), jubilee (= mumnoncitsi); the period of time may be short or long as indicated by x2, and may occur every year or every nth year as indicated by x3 (default every year); (x2 and/or x3 may need metaphorical restriction: djecitsi, pavdeicitsi; also equinox, solstice, time of year. See also cedra, crisa, critu, dunra, ranji, temci, vensa, jbena.

In notes:

x1 is an era/epoch/age characterized by x2 (event/property/interval/idea).
x1 is summer/summertime [hot season] of year x2 at location x3.
x1 is autumn/fall [harvest/cooling season] of year x2 at location x3.
x1 is winter/wintertime [cold season] of year x2 at location x3.
x1 is born to x2 at time x3 [birthday] and place x4 [birthplace]; x1 is native to (fo) x4.
x1 (event/state) continues/persists over interval x2; x1 (property - ka) is continuous over x2.
x1 is the time-duration/interval/period/[elapsed time] from time/event x2 to time/event x3.
x1 is spring/springtime [warming season] of year x2 at location x3; (adjective:) x1 is vernal.
ci1 is a term/semester with interval ci2 of year ci3 for school ck1.