You've added the check box to the left of the date so it is actually in Cells[0].
If you remove the CheckBox then the date again becomes Cells[0].
You have a choice now ...
Either put the CheckBox column to the right of the date (Cells[1])
OR
change the line
DGVCalendar.Rows[row].Cells[0].Value
to be
DGVCalendar.Rows[row].Cells[1].Value
Better yet, define some const values e.g.
const int DateColumn = 0
and use
DGVCalendar.Rows[row].Cells[DateColumn].Value