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