"Sent a letter by Muggle post telling them they'd been short-listed for the All-England Best Kept Suburban Lawn Competition. They're heading off to the prize-giving right now ... or they think they are."
-- Nymphadora Tonks
All-England Best Kept Suburban Lawn Competition is an invented contest; the Order of the Phoenix used this as a ploy to get the Dursleys out of the house, so members of the Order could come and take Harry Potter safely to Grimmauld Place (OP3).
It was Tonks’ idea to use a lawn competition as a diversion, sending a letter to the Dursleys by muggle post (OP3).
"Harry had a fleeting vision of Uncle Vernon's face when he realised there was no All-England Best Kept Suburban Lawn Competition."
"Two of those tiny lights might belong to Uncle Vernon's car ... the Dursleys would be heading back to their empty house right now, full of rage about the non-existent Lawn Competition... and Harry laughed aloud at the thought"
There was a drought in the Southeast of England this summer:
"Cars that were usually gleaming stood dusty in their drives and lawns that were once emerald green lay parched and yellowing - for the use of hosepipes had been banned due to drought" (OP1)
Harry overhears the Dursleys discussing the news which reports on the drought:
"('I hope he's listening next door!' bellowed Uncle Vernon. 'Him with his sprinklers on at three in the morning!')"
Though the drought has affected the Dursleys' lawns - there is a mention of "Aunt Petunia's dying begonias" (OP1) - one may question whether or not the Dursleys are complying with the watering ban. Is the idea of Dursleys winning a lawn competition ironic because their lawn is dead, or is it because they are breaking the ban and their lawn is the best looking on Privet Drive?