Here are some theories to discuss, they seem pretty legit to me except the fifth theory, and part of the third.

1. Finn's Found/Birthday. We can see in "Mystery Train", that it is Finn's birthday, but if we look at the airdate, that's March the 14th, now, don't go all "So Motor, that's just airdate" and whazzows like that. It's 3/14/???? 3/14 = 3.14... π (Pi), and Finn used to say "Mathematical!". Geddit?

2. The Possible Mushroom War Date?

On the back of the Enchiridion, there are three numbers: 8, 13, and 21, August 13th, 2021. You may say this is the Fibonacci Sequence... But what day of the week with this be? Friday the 13th.

3. Origin of Raincorns.

The Mushroom War is meant to be maybe World War III. But right, the North Koreans are being stupid. Now if we think about it, Rainicorns speak Korean and like the taste of Humans, what does this mean? It may mean that the North Koreans wanted to create a flying animal that could blind their enemies with colour, so they tweaked the DNA of horses and trained the new Rainicorns to kill and eat other humans, but after the nuclear fallout, the Rainicorns broke free and ate all of the North Koreans.

4. Susan Strong is Finn's mother.

Well, in the alternate timeline, Finn's mother has blonde hair, and Susan has blonde hair. Also, when Susan first sees the sun, she is afraid of it, but Finn comforts her by tell her it is the sun, Susan then looks at him and says: "Sun?" or did she say: "Son?"

She has a similar body build to adult Finn too.

5. Susan Strong is Fionna, and Prince Gumball is Princess Bubblegum's Uncle.

So discuss! What do you think?