@candle Yup, that’s basically it. Again, just a turn of phrase you have to get used to without real explanation.
Although you could also say « j’en mange les lundis », it’s correct as well, but feels a bit… unnatural.
(I think there’s also a veery slight nuance in meaning, but I’m not sure and am also too tired to brain it anyway)

@candle nah, en is more like do "I do eat on mondays", like, sometimes, maybe
you're declaring you do it, but not like an exclusive thing like "I only eat on mondays"

@candle « en » is a pronoun refering to some amount of stuff you are talking about. With a context, those are equivalent :
« J'aime les pommes. Je mange des pommes le lundi. » (« I like apples. I eat apples on Mondays. »)
« J'aime les pommes. J'en mange le lundi. » (« I like apples. I eat some on Mondays. »)