Aliced,
Is there a way that you could be in San Sebastián by January 5, the eve before Epiphany?
If so, you could witness the festive Magi Kings' Parade, the Cabalgata de Reyes, which starts at 6 pm from Boulevard (the avenue that marks the border between the Old Quarter and the modern town.) It lasts for an hour. See the parade route here.

The Magi Kings arrive in the city in the morning of the 5th on the funicular of Monte Igeldo and greet the children waiting for them on the square at 11 am, then they spend the morning going around to the various neighborhoods, then at 2:30 they take a break in the City Hall where there is a reception at 3:30 when they children can give them their letters.
Then at 6 pm the parade of 5 floats begins--the floats of Joseph, Mary and baby Jesus followed by the floats of Melchor, Gaspar and Balthazar. After the parade the Magi Kings, los Reyes Magos, visit the senior centers.

On the 6th the children receive their gifts and families gather to enjoy their traditional Epiphany day treat, the Roscón de Reyes (the Kings' cake, a type of brioche). One of the best can be purchased at Oiartzun on Calle Ijentea.
https://www.eitb.eus/es/television/p...-roscon-reyes/

No, our travels will be from Bordeaux to Bayonne, staying there two nights and visiting nearby Biarritz too. Our return flight from CDG is on January 7 so cannot change that. Just ran out of time to consider San Sebastian. Thanks for all your input.

In 2018, we stayed in San Sebastián for 2 weeks studying Spanish, and on one Saturday we took the bus to Bilbao to see the Guggenheim and other nearby sites. Very easy to do. Did the same for Pamplona the following week. We left our car parked in SS.

I have been to Bayonne, Biarritz and San Sebastián on january. It is a fine season to visit these places but the weather can be tricky. Temperature can be somewhere around 10ºC (50ºF) with likely rain. Wind can be freezing cold, sky will be dark and cloudy.

if you happen to be in Bayonne on Jan 5th and interested in markets, dont miss the Quintaou market in Anglet. It is the biggest one in the area. Trambus #1 stops nearby.
if you like seaside walk, the most popular goes from La Barre (mouth of Adour river) to Chambre d'Amour. Again very easy with public transportation (#38 from Bayonne to La Barre).
2 days in Basque country is obviously not a lot but you may have time to visit few seaside villages or small towns like Guetary and St Jean de Luz. Both have local train services to Bayonne (or bus #3). Forget about the train for Biarritz as the station is located in the outskirts. Biarritz is maybe the only city in the world where the airport is closer to the city center than the train station 😀

Thank you, Rouelan. We will have January 5 as an open day: spend the day in Bayonne, or return to Paris a day early, so this might be the perfect option. Thx for tip as to Biarritz train station.