Something

    protected void TextBox1_TextChanged(object sender, EventArgs e)
    {
        int dayda = Convert.ToInt32(dayD.Text);
        if ((dayda <= 31) && (dayda >= 1))
        {
            day = dayda;
        }
        else
        {
            dayD.Text = "must be between 1 and 31";
        }
    }