public class Yearly :
    Monthly

Public Class Yearly
    Inherits Monthly

<dx:ASPxGantt ID="Gantt" runat="server" ... /> 
    ...
    <WorkTimeRules> 
        <dx:YearlyRule IsWorkDay="false">
            <Recurrence Day="27" Month="May" />
        </dx:YearlyRule>
        <dx:YearlyRule IsWorkDay="false">
            <Recurrence Day="14" Month="February" />
        </dx:YearlyRule>
        ...
    </WorkTimeRules>
</dx:ASPxGantt>

YearlyRule yearRule1 = new YearlyRule();
yearRule1.Recurrence.Day = 27;
yearRule1.Recurrence.Month = Month.May;
yearRule1.IsWorkDay = false;

YearlyRule yearRule2 = new YearlyRule();
yearRule2.Recurrence.Day = 14;
yearRule2.Recurrence.Month = Month.February;
yearRule2.IsWorkDay = false;

Gantt.WorkTimeRules.Add(yearRule1);
Gantt.WorkTimeRules.Add(yearRule2);