adverb/entɾeseˈmana/
on a weekday (the days Monday through Friday)
“El resto della concluían sayo de velarte, calzas de velludo para las fiestas, con sus pantuflos de lo mesmo, y los días de entresemana se honraba con su vellorí de lo más fino.” — The rest of it went in a doublet of fine cloth and velvet breeches and shoes to match for the holidays, while on week-days he made a brave figure in his best homespun.
From entre + semana.