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.
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:
If you completed a task at the last acceptable moment, then you completed it in the nick of time.
We use the expression train of thought to refer to the process of thinking about a sequence of connected ideas. We often use this expression to refer to an uninterrupted flow of ideas, or the development of a line of reasoning in our mind.