How to get hold on calendar dijit for xp:dateTimeHelper to set firstdayoftheweek attribute?

Question

I have set up a datepicker helper for an inputfield for my xpages form:

<xp:inputText id="agenda">
<xp:dateTimeHelper></xp:dateTimeHelper>
</xp:inputText>

I want to set the firstdayoftheweek attribute for the dojo calendar dijit but when I choose

var target = dojo.byId("#{id:agenda}")

I get hold of the input element

if I try

var target = dijit.byId("#{id:agenda}")

I get the hold on the same input element.

SO how should I get hold of the generated calendar dijit and then set the value for firstdayoftheweek to 1 (Monday?)