The locked cabinet on Day 3 takes 6294. Inside is the telescope.
Why this one matters more than it looks
The telescope is not a collectible. It is the last step of the game's final achievement.
The post-credits route for "You've found him!" ends with you taking the telescope to the window near the dining hall and looking at the upper window. If you finished the game without opening this cabinet, you will be loading a save to come back for it.
Open it on your first pass.
Where it is
Day 3, during the alchemy sequence. The other Day 3 lock in the same stretch is the alchemy box click order:
right, right, right, left, right, left, right, right, left
Both are on the all codes page and in context on the Day 3 walkthrough.
If 6294 is refused
The usual cause is scene progression rather than a wrong number. Several containers in this game will not accept input until the surrounding task has advanced far enough — finish the alchemy step you are on and try again.
The rest of Day 3
Three Day 3 arrangements — the cellar chest, the three-bottle order, and the fish placement for Final Touch — are only documented in the community as screenshots. We have not transcribed them because we cannot describe them accurately from an image alone. See the Day 3 walkthrough for what is confirmed.