I think that this sentence doesn't make any sense both grammatically and semantically. is this person using a different calendar? what does it mean "the Friday"?? shouldn't it be "on your calendar" (mothertongue speakers of English , please, tell me if I am wrong on tis)???
Native (American) English speaker here. I agree with you. We say "on your calendar". We say "in your schedule", though; "on your schedule" is also fine. It's possible to use use "calendar" and "schedule" as synonyms (It's on my calendar"; "It's on/in my schedule"), but even so, "on" is the preposition for calendar. "The Friday" doesn't make sense except in instances given in other replies.
A preferable translation would be "It is Friday on uour calendar". It's not "in your calendar" and it's not ""the" Friday". Obviously these are things which are expressed differently in Danish from English, eg in your calendar vs on your calendar, and/or things which aren't considered necessary in one language whilst still being included in the other, eg the Friday, the Spring - used in Danish, but "the" is commonly omitted in English. I wish we'd stop having to translate things literally, especially when the literal translation makes very little sense. It's a waste of time!
Here we go again!! I hate having to write a grammatically incorrect English answer, so that I can finish the Danish level!! We shouldn't have to look into the it's, what's, might be's and all other possibilities for a single sentence - we're here to learn a language which is made more difficult through the bad translations that we're continually supplied with. It's really quite far from satisfactory!