Spreadsheet Day was probably first celebrated in 2010.
Spreadsheets have been around since quite a while and are one of the main reasons that computers really caught on mostly in business administration.
The system of having a bunch of rows and columns that could be filled with heaps of data and formulas gave computers a huge edge over using actual paper. Because paper is static, while spreadsheets could be edited indefinitely and the computations would alter any data that was connected to other data if the formulas were declared right.
The first spreadsheet was probably LANPAR (LANguage for Programming Arrays at Random), which came into use in 1969. LANPAR was used on the huge computers that wouldn’t fit on a desk. Because computers back then were mostly HUGE!
In 1979 there finally was VisiCalc that enabled spreadsheets on personal computers. The kind that does fit on a desk.
We still use spreadsheets. Even though we now have programs and apps for most of the more specific boring computing.