It might be a bug. After little rnd i found that when I move your TextBox1 from above of the AttenCalendar control to bellow, then value is assigned and displayed.
when I use the bellow code
<div>
<asp:TextBox ID="txtId" runat="server"></asp:TextBox>
</div>
<div>
<asp:Calendar ID="AttenCalendar" runat="server" BackColor="White" BorderColor="SteelBlue"
BorderWidth="1px" Font-Names="Verdana" Font-Size="9pt" ForeColor="Black" Height="275px"
Width="100%" FirstDayOfWeek="Sunday" NextMonthText="Next >" PrevMonthText="< Prev"
NextPrevFormat="ShortMonth" ShowGridLines="false" BorderStyle="None" CssClass="monthCalendar"
OnDayRender="AttenCalendar_DayRender" SelectionMode="None" DayNameFormat="FirstLetter"
OtherMonthDayStyle-CssClass="otherMonthStyle" DayStyle-CssClass="dayStyle" DayHeaderStyle-CssClass="dayHeaderStyle"
WeekendDayStyle-CssClass="weekendDayStyle" TodayDayStyle-CssClass="todayStyle"
TitleStyle-CssClass="weekendDayStyle">
</asp:Calendar>
</div>
The textbox value is not assigned/displayed by DayRender event handler.
protected void AttenCalendar_DayRender(object sender, DayRenderEventArgs e)
{
txtId.Text ="hi";
}
But same thing is assigned when my markup like as follows(just textbox1 is bellow of Calender control)
<div>
<asp:Calendar ID="AttenCalendar" runat="server" BackColor="White" BorderColor="SteelBlue"
BorderWidth="1px" Font-Names="Verdana" Font-Size="9pt" ForeColor="Black" Height="275px"
Width="100%" FirstDayOfWeek="Sunday" NextMonthText="Next >" PrevMonthText="< Prev"
NextPrevFormat="ShortMonth" ShowGridLines="false" BorderStyle="None" CssClass="monthCalendar"
OnDayRender="AttenCalendar_DayRender" SelectionMode="None" DayNameFormat="FirstLetter"
OtherMonthDayStyle-CssClass="otherMonthStyle" DayStyle-CssClass="dayStyle" DayHeaderStyle-CssClass="dayHeaderStyle"
WeekendDayStyle-CssClass="weekendDayStyle" TodayDayStyle-CssClass="todayStyle"
TitleStyle-CssClass="weekendDayStyle">
</asp:Calendar>
</div>
<div>
<asp:TextBox ID="txtId" runat="server"></asp:TextBox>
</div>
It is assigned and displayed to my page.