extremely rare (idiom); once in a blue moon

to meet (by chance); to come across

to meet by chance; to come across; (of a calendar event) to come along; (of an event) to fall on (a particular day); to fawn upon

every time; on each occasion; whenever

on the happy occasion of

to just happen to coincide with

to encounter (sth unpleasant)

to meet again after a long period of separation

lit. to meet face to face on a narrow path (idiom); fig. enemies or rivals meet face to face

strangers coming together by chance (idiom)

it's a small world (idiom)