Once in a Blue Moon

History:
It does not happy very often, but occasionally there are two full moons in the same calendar month. The second full moon is called a Blue Moon.

Meaning:
This expression is used when referring to any event that happens very rarely.

Example:
My husband and I really enjoy going out for a nice steak dinner.  However, to keep the event a special one, we only go "once in a blue moon"

A Fly on the Wall

This idiom often baffles foreigners.

History:
The expression was first noticed in the 1920's, and it's first publication was  in the February 1922 edition of the Oakland Tribune.  Came into wide acceptance during The 1960's,

Meaning:
To be a "fly on the wall," means being able to see and hear what is happening around you without being noticed.  This reflects the fact that a fly on the wall generally goes unnoticed while activity proceeds around it.

Example:
When my husband and I finally admitted that we were officially dating, after finding each other mutually attractive for some time, our friend Ray expressed that he'd have liked to have been a fly on the wall when that took place!  Sorry, but some things are private!