The Special Series Pokemon GX pack opened a +EV window at 4:41 PM PT on September 2 and peaked at +5.0% EV by 4:50, about 9 minutes after open. The whole window ran 38 minutes before closing at 5:18 PM PT.
The pack sat at -0.3% EV in our last reading before the window. One minute later, at open, it had flipped to +0.9%. By peak it was at +5.0%, then it drifted back to -3.1% at close.
What actually moved EV
The pool held 53 cards throughout, so pool size wasn't the story. The three Grail-tier anchors (Arceus/Dialga/Palkia GX at $4,965, Latias & Latios GX at $4,814, and Mewtwo & Mew GX at $2,891) all kept identical odds (0.03% each) across every snapshot, contributing a combined $3.80 of EV that never changed.
The real movement was in the Lineup tier. Between before_window and at_peak, a cluster of high-value cards cycled in and out:
Cards that entered the pool by at_peak and weren't there before: An Eevee & Snorlax GX (2019 S&M) worth $238 joined at 2.0% odds, adding $4.75 in EV contribution. A second Glaceon GX (2018 S&M) at $238 appeared alongside the existing 2019 version, adding another $4.75. A Latias/Latios GX Lineup card (2019 S&M, $101, $2.03 EV contribution) entered, as did additional Charizard GX and Celebi & Venusaur GX copies, and several other cards in the $60 to $100 value range.
Cards that were pulled out between before_window and at_peak: Kommo-o GX ($122 value, $2.43 EV contribution), Gyarados GX 2017 ($100, $2.01), Venomoth GX ($93, $1.87), Scizor GX ($87, $1.74), Dragonite GX 2018 S&M ($81, $1.62), and a Guzzlord GX and Eevee & Snorlax GX (2018 JPN) among others moved out as the pool reshuffled into a fresh lineup.
The net effect of those card-level changes pushed total EV contribution meaningfully upward from the before_window reading (-0.3%) to the peak (5.0%), a swing of roughly 5.3 percentage points on a $100 pack.
Between peak and close, several of the stronger Lineup cards (the higher-value Umbreon GX 2019 S&M, both Glaceon GX 2019 entries, Rowlet & Alolan Exeggutor GX at $162, Blastoise & Piplup GX, both Sigilyph GX copies, Kommo-o GX, multiple Charizard GX, and others) were pulled out of the pool, and a different set of Lineup cards replaced them. The incoming cards carried lower values on average, which pulled EV back below zero by the 5:18 PM close.
The window lasted 38 minutes, peak was reached in under 10. At that peak, a $100 pack carried just over $105 in weighted expected value by our reading.