I think that there are many people who know their birthday.
Do you know to what day of the week it was born?
If a birth date is inputted, it will be an application which the day understands what day of the week is.
How many persons of the same day-of-the-week birth are in a class or an office?
In addition, if the year of the future is inputted, the day of the week of the next birthday will also be known.