Does anyone know which weekend will be the bridge for constitution day/immaculate conception. One is on Tuesday the 6th and the other Thursday the 8th.

I have a long-ago booked frequent flyer ticket to Paris in December and for obvious reasons I am considering other options. (I like to stay between Place de Republique and the northern train stations and this is already becoming a hot spot.)

I can get a round trip ticket on Air France to Madrid for US$111 and whisk off to Madrid the day I arrive, but I don't want to repeat the crowds of last year.

Any advice?

I'm not sure I understand you...or I don't know if you understand th "philosophy" of puentes.
The obvious answer to your question is...BOTH!
Some people will go out friday to tuesday, others will do thursday to monday .... an even some others will do the "ACUEDUCTO", firday to the next monday, the full week.

schools will close the friday