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