Ginny Weasley was wet. Sopping, soaking, wringing wet. And rather pissed, to tell the truth.
With the onset of her brother's new invention, the Homing Balloon, she literally couldn't hide from the stupid water balloons. And since it was summer, she couldn't shield herself, or even perform drying charms. She would have told her Mum, but she knew that the second a word of the balloons touched her lips in the vicinity of her mother, the twins would shout from the rooftop that they had caught her and Harry kissing at the end of last term.
Her Mum probably wouldn't be that upset, but Ginny really didn't want to have to deal with her mother's incessant love advice, especially with Harry. And Bill or Charlie would probably come home and "warn" Harry about being a gentleman.
Ron had just turned seventeen, and could have helped her get back at the twins, but he was still miffed about her accidentally slipping and mentioning Hermione in front of Mum- which was, coincidentally, how she was sure she'd be inundated with "helpful" advice if her mother learned about Harry.
Sometimes it really bit, being the youngest.
But anyways, back to the original problem. Ginny Weasley was wet. And Harry was due to arrive by Portkey any second now, for his birthday party and- oh!
Harry was of age now, wasn't he?
Hello, revenge.