una golondrina no hace verano
Spanish edit
Etymology edit
Literally, “one swallow doesn't make a summer”, a calque of Ancient Greek μία χελιδὼν ἔαρ οὐ ποιεῖ (mía khelidṑn éar ou poieî, “one swallow does not make a spring”); the reference to summer instead of spring is shared with various other European languages (see the descendants at the Greek entry).