What is the Feast of the Assumption?

From the text:

‘Today is the Feast of the Assumption. August fifteenth. Don’t tell me you never heard that.’ Oh, sure, the Feast ofthe Assumption—Brother Gerald preached on that every other Sunday. Of course I’d never heard of it. I shook my head.
‘We didn’t really allow Mary at our church except at Christmas.’
August smiled and dunked a wooden drizzle into the vat of honey, which sat on the counter by the toaster oven. While she spun honey across the tops of a fresh pan of cakes, she explained to me in detail how the Assumption was nothing less than Mary rising up to heaven. Mary died and woke up, and the angels carried her up there in swirling clouds.