Why does the calendar shift one day each year?

I assume you mean something like "why was Dec 31 a Wednesday last year and a Thursday this year"? There are 365 days in a year, however this does not divide evenly with the number of days in a week. 7 days/week x 52 weeks = 364 days. Therefore there's an extra "day of the week" added each year. On leap years an additional day is added.