You can do it as below in clientside code.
<form id="form1" runat="server">
<div>
<select id="Select1" name="D1" önchange="DataEntryPanel.style.display='block'">
<option selected="selected" id="">Please Select</option>
<option id="Wedding">Wedding</option>
<option id="Lunch">Lunch</option>
</select><br /><br />
<div id="DataEntryPanel" style="display: none;">
Event Name <input id="Text1" type="text" /><br /><br />
Time <input id="Text2" type="text" /><br /><br />
Location <input id="Text3" type="text" /><br /><br />
<input type="submit id="btnSubmit" name="btnSubmit" value="Submit" />
</div>
</div>
</form>