‘I’m bored!‘ said Billy Badger.
This point applies mostly to children’s writing. There’s technically nothing wrong with starting a story or novel with ‘I’m bored’/Billy Badger was bored – but it’s been done to death and probably won’t get you off on the right footing with an agent or editor. I knew I’d even seen a whole book called I’m Bored, so I checked and found five, together with several variations on the theme.
I would also put openings like Monday started like a perfectly normal day/any other day… into the same category. Again, not ‘wrong’ but unoriginal bordering on cliché.
Scribble stuff down like this to get yourself going, by all means. I’m an advocate of not self-editing when writing a first draft because I think it spoils the creative flow (even though I have trouble following my own advice – it’s very tempting to keep stopping and tinkering!) But look out for stuff like this when you go back and read through because to my mind it will tend to mark you out as an inexperienced writer.