score:5
According to the DHS, it usually takes 72 hours for an ESTA application to be processed. Nothing is ever guaranteed, but there should be plenty of time to get this accomplished before Christmas, though it's a busy travel time, so airplane fares may be high if you haven't bought your ticket yet.
A valid Spanish passport (with at least six months of validity remaining) and approved ESTA are enough to enter the US in normal times; see our FAQ on covid requirements to make sure your vaccine proof meets the requirements (it probably does) and be sure to have plans to get a covid test the day you plan to travel or the day before.
Upvote:2
Also, do I need/is recommended to get a health insurance?
Yes. Medical travel insurance is cheap if you restrict it to just medical and medical costs in the US are the highest in the world and very unpredictable.
The key is to restrict the insurance to medical only and do not get coverage for travel cost (cancellation, refunds, delay, lost baggage, etc.) The latter ones tend to be very expensive and have a lots of complicated exclusions.