"Last month, the synagogue was under construction."
Translation:Le mois dernier, la synagogue était en travaux.
A more specific answer to your question is that adjectives that are not in the acronym BANGS are generally placed after the noun, but can be placed before the noun to give it a figurative meaning. You can say « le dernier mois », but it figuratively means "the last (as in final) month." « Le mois dernier » literally means "last (as in the previous) month".