once in a purple moon

all at once

Unexpectedly; without warning; all of a sudden.

ask for the moon

To claim or desire something that one cannot have.

blue moon

A long time.

for this once

On this particular occasion, to make an exception

hang the moon

To place the moon in the sky: used as an example of a superlative act attributed to someone viewed with uncritical or excessive awe, reverence, or infatuation.

howl at the moon

To spend considerable time and effort fruitlessly in a hopeless endeavor.

once again

Again, once more, one more time.

once and for all

Conclusively, finally, permanently.