I need to know the date of the last Monday

<cfset todayDayNumber  = dayOfWeek(now())>

<!--- since Monday's day-of-week is 2 --->
<cfset lastMondayRelativeDayNumber = todayDayNumber - 2>

<!--- adjust if today is Sunday --->
<cfif todayDayNumber is 1>
    <cfset lastMondayRelativeDayNumber = 6>
</cfif>

<cfset lastMonday = dateAdd("d",-lastMondayRelativeDayNumber,now())>

the day() function will tell you what day of now() is as an integer: 1=Sunday 7=Saturaday.

You know that Monday is 2.  The difference between today's number and Monday's number (with a little mathmatical juggling to wrap Sunday to the previous week  -- I would probably just convert -1 to 6 with a simple if statement after substracting 2 from the day() value.).

Subtracting (adding a negative value) of this difference with the dateAdd() function will give you the date of that previous Monday.