Mike and Will met on the Swingset on September 7th, 1976
First of all, all these observations were mostly made in my groupchat Michael's Flipflops! A shoutout to them. We named these strings of observations 'sevengate'.
It's already canon that they met on the swings on September 1976, according to the Wiki. But we didn't know the specific day until Ash cracked the code.
"It was the first day of Kindergarten", Mike told us himself in his S2 monologue.
The only possible day that they could meet is September 7th 1976, a Tuesday. School usually started after Labor Day up until the early 90s.
And we all know why the day they met being specifically a seven is full of meaning...
That isn't the only thing. 1976 + 7 = 1983, when Mike lost Will. Because he was kidnapped.
According to chinese astrology - "A trip to china sounds nice", 1976 is the year of the "Fire Dragon", and remember what Will painted for Mike?
Also adicional credits to Connor, who also cracked some parts.