PLEASE NOTE: Most flights from mainland USA to Spain depart in the afternoon, arriving in Spain the following morning. So, if you plan to arrive in San Sebastian on Thursday August 1, 2019 you would need to fly out of the USA on Wednesday July 31, 2019.
I suggest arriving in San Sebastian by Thursday August 1, 2019 in order to allow yourself plenty of time to recover from jetlag and to acclimate to paradise.
There are no direct flights to San Sebastian. Most flights to San Sebastian connect through Madrid. Please Note: Most flights depart the USA in the late afternoon and arrive in Spain the next morning.
There are also inexpensive flights on TAP Air Portugal if you search on Kayak.com. I have never flown TAP Air Portugal but it seems like an affordable way to at least get to Madrid and then book a separate flight to San Sebastian
Iberia has roundtrip flights from Madrid to San Sebastian.
San Sebastian is a 4.5 hour drive from Madrid and a 5.5 hour drive from Barcelona. Should you choose to drive, remember that most cars in Europe have a stick shift. There is no train from Madrid to San Sebastian, but there is a bus. The shortest bus ride is about 5hr 30min from Madrid to San Sebastian. There is a train from Barcelona but I have never taken it before.
THE JEWEL OF BASQUE COUNTRY
Here are some articles about San Sebastian:
Here is a fabulous New York Times' article about San Sebástian with great ideas of things to see and do during your visit.
Here is an Article in Budget Traveler featuring 40 photos that will convince you to travel to San Sebastián.
https://www.travelsewhere.net/things-to-do-in-san-sebastian/
And here is an Article with a list of 7 things to do while in San Sebastián.