Each year has 365 days or 52 weeks plus one day, and each week has two weekend days, which means there are approximately 104 weekend days each year. Factors that affect this number are leap years, the day on which the year changes and the "plus one" day.
The idea of the year was created to encompass the four seasonal changes that the Earth goes through. A calendar year is actually 365.24 days based on Earth's orbit around the sun. Leap years occur once every four years to account for the fractional time accrued over the course of four years.