Here is the full list of all 21 Randal's Monday achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Beat the game without using the hints system.
Beat the final conversation choosing only the correct answers.
Beat the hot plate puzzle on your first try.
Tell Bogs the password.
You didn’t fail any question in HAL’s game.
Beat the puzzle of the drawers and the books on your first try.
Feed the pigeons three times.
Beat the whole game.
Feed the Horsemen.
Complete episode 6.
You made a complete scarecrow.
Complete episode 5.
Complete episode 4.
Complete episode 3.
Hit the “kill kitty” button 20 times.
Complete episode 2.
Beat the puzzle of the Sci-Fi Con sign without the bum’s help.
Complete episode 1.
Unlock the hints system.
Sell the ring.
Complete the prologue.