In Harry Potter and the Philosopher's Stone, the trio attend Nearly Headless Nick's Deathday party (something they left out of the movies, unfortunately.)
The party falls on October 31st, 1992, which is Halloween duh! Awesome day for a party like that right? Well yeah, but also nahhhh.
Why? Because October 31st, 1981 was the night Harry's parent's died at the hands of Lord Voldemort. So... On the anniversary of his parents' death, Harry is attending a Deathday party. Like we said, grim.
They state, "It's even more interesting considering that when Dumbledore and McGonagall initially drop Harry off at the Dursleys' house all those years ago, McGonagall says: 'This boy will be famous, a legend. I wouldn't be surprised if today was known as Harry Potter day in the future'".
As we know, it wasn't...
The user went on to explain that, "So the fact that just 10 years on, the whole Wizarding World has seemingly forgotten about marking the day on which Lord Voldemort was defeated, despite the fact that it ended the war and that we see in the first few chapters of Philosopher's Stone how much it meant to them at the time, is more strange to me than Harry not knowing when it parents died."
Weird. Very weird.
BUT - according to the Harry Potter Wikia page on Halloween, a lot of other very important events went down on October 31st throughout the series.
The annual Hallowe'en feast, the day the Chamber of Secrets was opened, and the day Sirius Black attacked the Fat Lady.
Although, it's important to note that it isn't 'til the very last book that we learn the Potters' died on that particular date, so maybe J.K. wasn't aware of it while writing the first book?
All we can say is Halloween ain't fun in the wizarding world :(