On May 6, 1955, IBM publicly demonstrated its new invention, disk storage. The disk drive would hold far more data at lower cost than the existing magnetic drum.
The disk drive allowed for information to accessed immediately since the disk arm went directly to the location of the data. With magnetic tape, the tape file must be sequentially read record by record to find the desired data.
The initial disk drive was huge and contained 5 million characters. Improvements got that up to 50 million.
The drive was formally announced as a product in September, 1956. The disk was 350 and was part of the 305 system. The 350 disk was later enabled to attached to existing and upcoming IBM computers.
[I don't know what IBM considers the "official" anniversary date.]For the first 30 years, disk drives remained expensive and limited to critical information. But soon the price dropped and capacity climbed making them very cheap even for home use.
The Internet could not exist today without the disk drive which stores everything we want to see.