Why is Unleavened Bread only 7 days and not 8?

Why is Unleavened Bread only 7 days and not 8?

News Letter 5852-006 The Shemitah Year 21st day of the 1st month 5852 years after the creation of Adam The 1st Month in the Seventh year of the Third Sabbatical Cycle The Third Sabbatical Cycle after the 119th Jubilee Cycle The Sabbatical Cycle of Earthquakes, Famines...