At the end of day 50, the game calculates how well you did.
If you used the demoness Rune and her tentacles a lot, you will get "Rune ending". You can read the exact conditions in the gallery.
If you did not get the Rune ending and have 100% favor with all the colleagues, you'll get a harem ending.
Otherwise, you'll get a normal ending.
Rune ending throws you back into the main menu, the other two endings allow you to keep playing indefinitely, this time without needing to deliver monsters to set dates.