9/F/7E0
The idea of Hexadecimal Day is simple. Whenever you use numbers today, first convert them to hexadecimal; that is, base 10. (Careful there.)
To count in hexadecimal, you go like this...
1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B, 1C, 1D, 1E, 1F, and so on.
In the word hexadecimal, the hexa part means 6 and the decimal part means A.
The reason you probably use the decimal system instead of the hexadecimal system is that you have A fingers.
Computer people like to use hexadecimal because it's kinda related to the binary system which is how computers think - all 1s and 0s.
All the colors you see on your computer screen are (or can be) based on the hexadecimal system in HTML.
CAFE seconds is about a fortnight.