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 puzzle of the drawers and the books on your first try.
Hit the “kill kitty” button 20 times.
Unlock the hints system.
Beat the game without using the hints system.
Beat the whole game.
Beat the final conversation choosing only the correct answers.
Feed the Horsemen.
Complete episode 6.
You made a complete scarecrow.
Beat the hot plate puzzle on your first try.
Tell Bogs the password.
Complete episode 5.
Complete episode 4.
You didn’t fail any question in HAL’s game.
Complete episode 3.
Complete episode 2.
Beat the puzzle of the Sci-Fi Con sign without the bum’s help.
Feed the pigeons three times.
Complete episode 1.
Sell the ring.
Complete the prologue.