The idiom "once in a blue moon" means:

Explanation

The idiom 'once in a blue moon' means that something happens very rarely. The term 'blue moon' refers to a rare astronomical event when a second full moon occurs within a single calendar month, which is infrequent. Therefore, the correct answer is 'very rarely'. Other options are incorrect because 'very frequently' and 'every day' imply a high frequency of occurrence, which contradicts the meaning of the idiom, while 'during the night' does not relate to the frequency aspect at all.