As the name implies, create a holiday.
Explain what you're celebrating, why you're celebrating it, and a date for it to be on. The parameters are optional, so just have fun with it.
Feel free to expand on anything, or just throw out pointless holidays (It's not like I can stop you)
Examples:
T-Shirt Day: Everyone wear's a T-Shirt that day.
Unplugging Day: No one uses electronics, this way we have more respect for them.
Google Day: Celebrate Google and it's knowledge on 4/11.
Day of Silence: A day without music