Suggestion on custom validation on SharePoint Online list

Hello,

I have a list on SharePoint Online where there are 2 date/time fields that I use to enter extra work time; I'd like to do a validationon these fields and I suppose that I have to write some code; the validation is on date (if it is a working day, sunday or an holiday like Christmas, Easter, etc.) and on the hours (signal that for example the user inserted an hour that is not in the extrawork timeframe).

Because I have no previous experience on use code to validate something, I'd like to know what is recommended to do now on the "Online" side and how to start quickly and is proficient in short time.

I've seen some suggestion aboun jquery, but also in the past: I don't know if it is good on the SPO, it is more used on the "On Premise" side.

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Regards