Sequential calendar dates occur when the date is read in a sequence, whether it is increasing or decreasing.  If the date format is read as DD/MM/YY, then 01/02/03, 02/03/04, 03/04/05, 11/12/13 etc are the Sequential Dates.

The first sequential day in more than twenty years occurred on August 9, 2010 at 12:34:56.7pm. This day was known as Sequential day because the rare numerical phenomenon where the time and date read 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 if read as 12:34:56.7pm 08/09/10. However this only occurred for areas that read the date in the format of MM/DD/YY, otherwise this phenomenon would occur on September 8, 2010, if the date format is read as DD/MM/YY.