How can I choose a date of a month so that a notion task is repeated, on a monthly basis?
What happens is that it can't find the way to do it and also it doesn't give me the option to do it like this :(
In your tasks database create a template with recurring options.
Click the little arrow at the right of the "New" blue button and create the task that will be repeating.
Go back and click again the little arrow and you will see your template there, click the three dots to see the options, there you will see recurring options.
That will do exactly what you need to have recurring tasks.