What should I do when my validity period has been accidentally activated but I don't want to use it?

Somewhat off-topic but if you wish to travel around Christmas & New Year I'd recommend booking seat reservations ASAP to avoid disappointment (when required at least).

If you'd like some advice post your plan and we'll gladly help.

It does normally mean that you went through the entire activation procedure and explicitly chose 25 November as the start date of the pass validity.

That's really unfortunate. It's easy to make a mistake with the month if you're not looking closely. I've done the same with a flight in the past, and arrived a month late.

According to the terms, this can't be refunded, unfortunately. You could try asking customer service if there's anything they can do, as @Al_G suggested.

https://www.eurail.com/en/eurail-passes/ordering-info/refund-exchange-policy

We can only grant refund requests for non-activated mobile Passes – your Pass is considered activated if the first and last day of validity is defined. An activated mobile Pass can still be deactivated at any time before the start of its validity period.

This is one of the reason why you shouldn't activate the pass until just before boarding the first train of your travel. Then there's no risk that you activate the pass on the wrong month.