Monday's ordinary thing is water under the bridge. The saying refers to a thing that is gone, or an incident that is done. It is irrevocable. Water under the bridge cannot be called back. Never to return, it bears its droplets away to bigger streams, rivers, and out to sea.
Is there an event you sigh over and say, "That's all water under the bridge now"--? Ah, but although the thing has happened just as it has, it can return to you in another form: in a memory, a story, or perhaps a poem or drawing.
What is your water under the bridge?