From what I understand you want to fill the datagrid in the list of days per month.
You can use the static method DaysInMonth(int year, int month) in the DateTime class to get the number of days the requested month has.
From there you can build a collection of strings (or whatever).
int daysInMonth = DateTime.DaysInMonth(2013, 2);
string daysCollection = new string[daysInMonth];
for (int index = 0; index < daysInMonth; index++)
daysCollection[index] = (index + 1).ToString() + "/2/2013";
Now you can fill your datagrid data source with the collection you just created