C#

if(year % 4 == 0)
    //is leap year
else
    //not leap year

if(year % 4 == 0 && year % 100 == 0 ? year % 400 == 0 : true)//hope you get that logic
   //is leap year
else
   //not leap year

if (DateTime.IsLeapYear(year)) {
   // is leap year
} else {
   // not leap year
}

int daysInMonth = DateTime.DaysInMonth(2009, 2);
Console.WriteLine(daysInMonth);

private void button1_Click(object sender, EventArgs e) {
	button1.Enabled = false;
	int year = dateTimePicker1.Value.Year;
	int month = dateTimePicker1.Value.Month;
	DateTime start = new DateTime(year, month, 1);
	dateTimePicker1.Value = start;
	int days = DateTime.DaysInMonth(year, month);
	for (int i = 0; i < days; i++) {
		dataGridView1.Rows.Add();
		dataGridView1.Rows[i].Cells[0].Value = start.AddDays(i).DayOfWeek;
		dataGridView1.Rows[i].Cells[1].Value = start.AddDays(i).ToShortDateString();
	}
}