I have so many questions.

Is the
#Fensterfreitag only on #Mastodon or is it in the entire #Fediverse?

Is it in
#Germany, only, or is there an international #WindowFriday, too?

Is the photo supposed to be taken today or can it be a few weeks old?

Does it have to be directly posted here, or can it be a link to a web page with that photo?

What happens if I
#quotepost on #Calckey? Is it properly resolved by Mastodon clients?

Is the photo from the linked web page shown? Or the photo from the quoted post?

I just have to test: So here's a link to
#ipernity in a quote post on Calkkey: http://www.ipernity.com/doc/rumpel/51938200

RE:
https://mstdn.strafpla.net/users/Muellers_Kabinett/statuses/110473967729363282
Mal ordentlich lüften

By York. Es wird sogar so warm, dass wir nicht nur die Jacke, sondern auch ein Fensterchen öffnen können.

ipernity
LOL. The #Mastodon client shows the #WindowFriday photo from the linked web page on #ipernity but links it to #calckey (!). It also places a link to the quoted post in the text behind a RE:. The link to the web page where the photo is from is lost. (Don't ask me how it should be. I haven't thought about that. I just made a test and observed something unexpected.)
This behavior seems to be specific for @MonaApp, though, because for example @tootapp correctly links the photo to where it's from (ipernity). After some thought that is what I would have expected, too, because that's how it was originally posted on Calckey. (The native Mastodon web app also keeps the correct link.) So it's probably just a bug in #Mona.