Order of pre-show events

Post Season 5/Siege of the Crystal Empire Update

Birth of Star Swirl.
Birth of the changelings.
Chrysalis witnesses umbrum rule over Crystal Empire.
Crystal Heart is taken from a dragon's hoard and the umbrum are sealed away.
The events leading up to and including the founding of Equestria as depicted in Hearth's Warming Eve.
The coronation of the alicorns, building of their castle, first contact with zebras, peace with griffons, and obtaining of sun and moon cutie marks as chronicled in The Journal of the Two Sisters
(Headcanon) Alicorn Sisters become adoptive aunts of Princess Platinum's heir, the ancestor of Blueblood.
The Events of Fiendship is Magic #2 with a young Tirek; among other reasons, Discord is there. (Unless it was another draconequus).
Discord invades, Battle of Discord permanently alters landscape in small area of Badlands, the Elements of Harmony are taken from the tree and used for the first time, plunderseeds planted.
Scorpan befriends a "young" Star Swirl, warns princesses of Tirek, resulting in imprisonment for Tirek, presumed ascension to throne for Scorpan.
Sombra is born.
Flashback events of Reflection arc of main comics.
Ascension, reign, and banishment of King Sombra, loss of Crystal Empire, Fall of Timbucktu, Events of Fiendship is Magic #3 with Sirens (Unknown order)
Radiant Hope makes contact with and lives among the umbrum.
Banishment of Nightmare Moon. Subsequent founding of E.U.P. Guard
1 year later, founding of Wonderbolts
Events of Fiendship is Magic #4 with Nightmare Moon and the Nyx
Changeling attack on Trot, holes put in changelings by furious Celestia, banishment for centuries in Volcano, (Escape presumably took place relatively shortly before season 2 finale), Yakyakistan closes its borders, Grub Hooffield & Piles McColt settle Smokey Mountains and begin feuding. (Unknown order)
Birth of current eldest generation of ponies, Founding of Ponyville.
The reign of King Guto, then the end of the Griffonstone monarchy.
Birth of current adult generation of ponies, Founding of frontier towns like Appleoosa. (Unknown order)
Cadence's origin story as told in Crystal Heart Spell
Birth of latest generation of ponies, including Mane 6.
100 x ? moons before Apple Family Reunion: Mane 6 are foals in diapers
Events of Neigh Anything arc
Previous Friendship Games
30 x ? moons before EQ: Fall of Sunset Shimmer (You can see blank flank Twilight with Shining and Cadence in background), Birth of school-age ponies, including CMC; Sunburst gains cutie mark and abandons Starlight Glimmer. (Unknown order)
Fillydelphia hosts the Equestria Games, Rainbow Dash meets and befriends Gilda. (Unknown order)
Events told in episode Cutie Mark Chronicles
Cheese Sandwich gains cutie mark, Charity Heart's community theater closes, Applejack & Coloratura attend Camp Friendship. (Unknown order)
Sweetie Belle's 5th birthday
Moon Dancer sets up "a little get-together at the west castle courtyard" and expected Twilight would attend. (Literally the last major pre-show event)

Headcanon note: Moons are probably either close to a month but not quite, or a half-month based on the time between the new and full moon as opposed to the full cycle.