Translation:Last month, the synagogue was under construction.
It seems that en travaux should translate to "in repair", which means the same as under construction.
Repair: Le mois dernier, la synagogue était en réparation. Build: Le mois dernier, la synagogue était en travaux.
"Under construction" can mean "being built" as well as "being repaired". It is the best translation as it covers both senses.