Someone

Option Explicit On
Option Strict On

Public Class MainForm

    Private Sub exitButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles exitButton.Click
        Me.Close()
    End Sub

    Private Sub monthTextBox_Enter(ByVal sender As Object, ByVal e As System.EventArgs) Handles monthTextBox.Enter
        monthTextBox.SelectAll()
    End Sub

    Private Sub monthTextBox_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles monthTextBox.TextChanged
        daysLabel.Text = String.Empty
    End Sub

    Private Sub displayButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles displayButton.Click
        Dim month() As Integer = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}
        Dim day() As Integer = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
        Dim num As Integer
        Dim isconverted As Boolean

        isconverted = Integer.TryParse(monthTextBox.Text, num)

        If isconverted AndAlso num >= 1 AndAlso num <= 12 Then
            daysLabel.Text = Convert.ToString(day(num - 1))
            MessageBox.Show("invalid month number", "Months", MessageBoxButtons.OK, MessageBoxIcon.Information)
        End If
    End Sub
End Class