Prime Days have been in June only one other time, in 2021.
My guess is that in 2025 July Prime Days out-performed Q4’s November’s BFCM week and/or the odd October Prime whatever-it-was–in whatever way is meaningful to Amazon but not necessarily obvious to onlookers–so they are moving Prime Days back to test if it was merely event proximity or sales oversaturation, versus an actual consumer shopping shift.
Or maybe Amazon just likes June better