When did Neville discover he wasn't actually a squib, when Uncle Algie dropped him out of the window and he bounced down the road? Year preferably, or a more specific date
The quote from the book is that it happened when he was eight, three years before the start of the first novel which takes place in 1991.
Since we don't know whether this was before or after the New Year (and given that we know that Neville was born on 30 July, 1980) that places the event as sometime between 31st July 1988 and 29th July 1989, operating on the principle that if it had happened on his birthday, he would have mentioned it.
My great-uncle Algie kept trying to catch me off my guard and force some magic out of me – he pushed me off the end of Blackpool pier once, I nearly drowned – but nothing happened until I was eight. Great-uncle Algie came round for tea and he was hanging me out of an upstairs window by the ankles when my great-auntie Enid offered him a meringue and he accidentally let go. But I bounced – all the way down the garden and into the road. They were all really pleased. Gran was crying, she was so happy. And you should have seen their faces when I got in here – they thought I might not be magic enough to come, you see. Great-uncle Algie was so pleased he bought me my toad.’ - HPPS: Chapter Seven - The Sorting Hat