When I was staying in Mexico in 1970 (!!), just after high school, I learned a saying:
Al nopal lo van a ver
Solo cuando tiene tunas.
They go see the prickly pear cactus
Only when it has prickly pears.
This is the context in which it was used: on Sundays, the young ladies of this small town would promenade in the plaza in one direction, and the young men in the opposite direction. (I don't know if this happens elsewhere, or how this got started, or if it still goes on.)
A group of not-so-young ladies joined the promenade -- these ladies were probably in their early 20s and still not married, a shocking situation at that time.
The young men paid attention to the teen-age girls but not to the "solteras" (old maids). Not a nice term, but that was the label that was used.
I was standing with a group of older men, who alerted me to what was going on. One of them trotted out that "dicho" (saying or proverb) about the tunas.