This day is long. was not accepted. Is هٰذا defining the day or the verb is? هٰذا يَوْم seems to mean This day.
"This day is long" would be هذا اليوم طويل (notice the addition of AL to يوم).
If we take it backward from the English sentence:
This is a long day: Here, the verb (to be) which is "is" separates the subject and the predicative (the predicative the information we are about to be told about or receive in general). Thus, the information provided is (a long day) = يوم طويل.
This day is long on the other hand, has "is" separating between "this day" and "long". Moreover, "This" defines "Day" and hence, the information we are about to receive is simply long (طويل). Hence, in Arabic, this would be هذا اليوم طويل with defining يوم to become اليوم.
This post I've made a while ago might help: https://forum.duolingo.com/comment/33483445