I can't guarantee this info is correct but I'm more inclined to believe it than not.....
I've heard that Glastonbury is considering happening in September next year if it doesn't feel confident about going ahead in June, and that it will decide which date in January.
So Weather in September on the face of it doesn't look too different from June. Temperature averages about the same.
Let's actually look back to the Friday-Sunday in previous years
First weekend in September
2019: Highs of 15, lows of 6. No rain
2018: Highs of 24, lows of 8. No rain.
2017:Highs of 20, lows of 10. Rain Sunday night
Second week in September
2019: Highs of 22, lows of 10. Rain Friday.
2018: Highs of 17, lows of 6. No rain.