I tried the game soon after the latest version was released and apparently it didn't have much. However, some things about it and the way the author acted made me think there was something more to it, so I told some friends about it and we began to investigate it.
Upon opening the executable, a logo with the title appears after which, as expected from a YNFG, you start in a bedroom. There's a radio you can interact with and a light switch you can turn ON and OFF, but you cannot sleep. From there you can go to the balcony, where you can see the moon, or through the door, which leads to a corridor. At the end of the corridor there's another door, and after that a black, empty looping room which you apparently cannot escape from.
That is, in principle, all you can do. However, the game changes slightly each time you open it. Mainly, the visual style of the title logos at the beginning seem to be random, and so are the phases of the moon. The radio cannot be turned on in the first run, but it is possible to do so in the following ones.
The most important detail, however, is that during the first seconds after the game is opened, some numbers appear on the window caption before quickly switching to "Nocturne 191". Some of my first tries were "2459238:19:39:36" and "2459238:19:42:46". The three last numbers were the hour, minute and second at which I opened the game. When running it the next day, the first number was replaced by a "2459239", so it was not hard to guess that it represented the number of days. We would later find out that it was written in the Julian date system, more info here: https://en.wikipedia.org/wiki/Julian_day
This lets us know that the game is aware of when you open it, a detail that becomes even more important when you notice dates like these are all over the author's tumblr. Pic related is a good example, it was uploaded on October 31, 2020, and the featured date translates into January 21, 2021, the day the game was published, so they were foreshadowing the release for quite some time. The date in the .zip file of the game is the same, too. And above all, in the tumblr post that links to the game’s itch.io page there’s another date alongside a full moon emoji. Such date translates into January 28, 2021, which is a full moon night, and also the day I’m writing this post.
So upon finding out all of this, we theorized the game would "activate" at that date. We could have changed our PC’s date, but we didn’t want to "ruin the magic", so we chose to wait. And finally, the day came. We opened the game soon after 00:00 AM a few times, but nothing strange seemed to happen, so we dropped it for a while, thinking we might have been wrong and the date referred to a different event. But later we opened the game again, without expecting anything, and suddenly the logo had changed to "|o||||||" (the author's username) instead of the typical "Nocturne". We tried out things and found out that, if you turn off the light and then interact with the radio, it will break after messing around a bit with it and the music will fade off. When this happened we could no longer turn the lights back on. We then went to the corridor which this time was dark and longer than usual. When we got to the looping room it was red instead of black and a hellish music began to play. Here the player sometimes turned around and looked at us without giving it any inputs, and its movement gradually slowed down until the game closed itself.
When reopening the game it seemed normal again, but by retrying multiple times we managed to trigger this, let's call it "cursed mode", more times, it seems that it only happens when you get the full moon phase. The visual style of the logos varies here too and we once got a "191" instead of "|o||||||". The rest seemed to be the same. However, this morning we tried again and we haven't been able to trigger it, despite getting the moon to be full, so it seems that not only it triggers at a specific date, but also only at the night of that date, a full moon night. It's honestly a pretty interesting mechanic for hidden events, we were quite shocked when we got it for the first time. Makes me wonder about what other creative things could it be used for.