Buon venerdì a tutti! Today's Parola del Giorno is the expression ad ogni morte di papa, which means once in a blue moon (literally, it means at every Pope's death). You might also see the expression ad ogni morte di vescovo, too.1

Since popes are generally known to have long reigns and live long lives, choosing new ones doesn't happen with much frequency. The expression describes something that happens quite infrequently.

See the expression used below:

Ho finalmente capito che Marina non è interessata alla nostra amicizia. Non si fa mai sentire, mi telefona ad ogni morte di papa e spesso non risponde ai miei inviti. Smetto di chiamarla.

I finally understand that Marina wasn't interested in our friendship. She never gets in touch, phones me once in a blue moon and often does not respond to my calls. I will stop calling her.