Guys, can you give me some idea on how to validate the day if the selected year is a leap year in combo box.

Thank you in advance.

See if this helps.
1 ComboBox

Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        If Not Date.IsLeapYear(CInt(ComboBox1.SelectedItem)) Then MsgBox("Not a Leap Year.") Else MsgBox("Leap year.")
    End Sub

This link might be of use.