"I always have hangovers on Sundays!"
Translation:J'ai toujours mal aux cheveux, le dimanche !
You used the plural "dimanches" here, instead of the singular "dimanche". J'ai toujours mal aux cheveux, le dimanche !
Last lesson, my answer was rejected because I used 'le mercredi' for wednesdays instead of 'les mercredis,' and now we get this. If both answers are acceptable, then ACCEPT both answers.
"Le dimanche" means "very Sunday" or "on Sundays", that is: every 7th day of the week, no limitation.
"Les dimanches" uses "les" as specific and as a restriction: this expression cannot stand alone without further information on their specificity/limitation: les dimanches d'été = all Sundays in summer.