@OKMaybeNot
If it is a VERRRRRY loud petrol mower, then surely it would be annoying anytime?!
Would you prefer he did it at 6pm (with the noise and petrol fumes,) when you're having your evening meal? Or at 3pm-5pm when you're out in the garden sunbathing, or at lunchtime when you're eating, or during the England football match?
Personally, I think I would prefer it at 8pm. I can hear 4 or 5 people mowing right now - mostly petrol mowers as many houses round here have large gardens/huge lawns...... Not bothering me in the slightest. In fact as someone said, it's the sound of summer, and is a lovely sound!!! 
And why not a Sunday? Not everyone works Monday to Friday 9-5, and has the weekend off. Many people work weekends, and also weird/late/early shifts. My DH does nights - 10pm-7am.
Shall I say to all my neighbours that they can't mow their lawn, or do anything in the garden, or have any music playing, or have their kids playing out (on the 4 days in a row he is in work,) til after he gets up at 3.30pm?! 