'It's for some stupid, noble reason, isn't it?'
-- Ginny Weasley, when Harry said they had to stop seeing each other
Harry talks to Ginny after Dumbledore’s funeral and tells her that they need to break up so Voldemort can’t use her to get to him.
‘Voldemort uses people his enemies are close to. He’s already used you as bait once, and that was just because you’re my best friend’s sister. Think how much danger you’ll be in if we keep this up. He’ll know, he’ll find out. He’ll try and get to me through you.’
‘What if I don’t care?’ said Ginny fiercely.
‘I care,’ said Harry. ‘How do you think I’d feel if this was your funeral … and it was my fault …’ (HBP30)
Both of them know that it’s not permanent. Ginny’s conversation on his birthday a month later and the kiss she gives him as a gift make that perfectly clear.
The date is derived from that of Dumbledore's funeral, which in turn is derived from the date of his death, which we know is 30 days before Scrimgeour comes to the Burrow to bring Dumbledore's will to Harry, Ron, and Hermione.