If you want to get first day of next month , use DateTime structure with AddMonths( ) method. using this method you can get first day of next month. Now, create a new instance of DateTime structure with some parameter like DateTime(Int32, Int32, Int32). This parameterized method Initializes a new instance of the DateTime structure to the specified year, month, and day. Here you can pass integer 2 in day field. Now, you can delete 1 day from current DateTime object using AddDays (-1) method.

Source Code 

 <form id="form1" runat="server">
    <div>  
        <asp:Button ID="Button1"
        runat="server"
        BackColor="#66CCFF"
        onclick="Button1_Click"
        Text="Click" />  
    </div>
       <asp:Label ID="Label1"
       runat="server"
       BackColor="Yellow"
       Text="Label"></asp:Label>
    </form>

Code Behind

 protected void Button1_Click(object sender, EventArgs e)
        {            
            DateTime today = DateTime.Today;            
            DateTime tempDate = today.AddMonths(1);
            DateTime tempDate2 = new DateTime(tempDate.Year, tempDate.Month, 2);
            DateTime nextmonthfirstday = tempDate2.AddDays(-1);
            Label1.Text = "Today : " + today.ToLongDateString();
            Label1.Text += "<br /><br />next month first day = ";
            Label1.Text += nextmonthfirstday.ToLongDateString();

        }

Code Generate the following output