[DefaultValue(1)]
public int Day { get; set; }

<DefaultValue(1)>
Public Property Day As Integer

<dx:ASPxGantt ID="Gantt" runat="server" ...>
  ...
    <WorkTimeRules>
        <dx:MonthlyRule IsWorkDay="false">
            <Recurrence Day="15" />
        </dx:MonthlyRule>
    </WorkTimeRules>
</dx:ASPxGantt>

MonthlyRule monthlyRule = new MonthlyRule();
monthlyRule.IsWorkDay = false;
monthlyRule.Recurrence.Day = 15;
Gantt.WorkTimeRules.Add(monthlyRule);