A calendar shows the days and dates of each month of the year.
This changes every year.
Here is an example showing the days and dates in February 2012.
2012 is a leap year, so there is one extra day in this February month.
Notice how we add 7 to the dates as we go forward a week.
Look at the dates on Wednesdays which go up in sevens: 1, 8, 15, 22 and 29.