Monday hour one- when things vary

How do you schedule something according to Monday hour one that varies with the weather or is dependent on someone else’s schedule?

I work with plants and some of the work I do cannot be done if it’s raining. A forecast can be helpful but is not always accurate.

Another example is going out to dinner with my partner. He wants to play it by ear and not pick a day – but rather see which days feels right when it gets closer. How do I plan for things like this?