If it can both mean on sunday and on sundays, how do you make the difference between only next sunday and all sundays forever?
Here it means "every Sunday" (on Sundays); next Sunday is dimancon.
Read the explanation here: https://www.duolingo.com/skill/eo/Dates-and-Time
But on sunday was also accepted, and thanks
I think "on Sunday" can in English also be interpreted as "on Sundays" / "every Sunday".
"I go to church on Sunday."
"I go to the gym on Wednesday."