a blue moon的用法和样例：
例句
- Barry only visits his parents once in a blue moon.
巴里很难得去看望他父母一次。 - If the river overflows its banks, the garden gets flooded; but that only happens once in a blue moon.
河水一旦泛滥，这片园子就会淹没，但这种情况极为罕见。 - I see her once in a blue moon.
我难得看到她。
词汇搭配
- once in a blue moon 罕见的，极少的，难得...
