Take a rain check on something

On a stormy day, relax and take a rain check.

If you need to postpone or cancel plans with someone then you can take a rain check. We use this expression when we want to politely decline an invitation or reschedule a plan, but also imply that we would be interested in accepting another one in the future. When you take a rain check on something it lets the other person know that although you are unable to accept their offer or participate in an activity now, you are hopeful that it will work out next time. It shows a willingness to reschedule for a later date.

Once in a blue moon

If you don’t do an activity very often, then you can say that you only do it once in a blue moon. We use this expression to emphasize how rare or infrequent an activity is. The activity could be infrequent due to cost, convenience, preference, etc.

This expression is somewhat casual, so it is best suited for informal contexts. For example: