Hey Liark, try my custom tag <cf_weeknumber>. Make sure to place the code below in a file, name the file "weeknumber.cfm" and place it in the custom tag dir on your server. Good luck!
<!---
cf_weeknumber by IQof188
Custom tag will display the correct weeknumber.
Input is a valid date, output is a variable called weeknumber (big surprise...)
Usage: <cf_weeknumber date="01/02/2001">
To output it: <cfoutput>#weeknumber#</cfoutput>
--->
<cfparam name="attributes.date">
<cfset YearType= DayOfWeek("01/01/"&DatePart("yyyy",attributes.date))-2>
<cfif YearType LT 0>
<cfset YearType=YearType+7>
</cfif>
<cfset WeekNumber=Ceiling((DayOfYear(attributes.date)+YearType)/7)>
<cfswitch expression="#YearType#">
<cfcase value="2">
<cfif WeekNumber IS 53 AND NOT IsLeapYear(DatePart("yyyy",attributes.date))>
<cfset WeekNumber=1>
</cfif>
</cfcase>
<cfcase value="4">
<cfset WeekNumber=WeekNumber-1>
<cfif WeekNumber IS 0>
<cfset WeekNumber=53>
</cfif>
</cfcase>
<cfcase value="5">
<cfset WeekNumber=WeekNumber-1>
<cfif WeekNumber IS 0>
<cfif IsLeapYear(DatePart("yyyy",attributes.date)-1)>
<cfset WeekNumber=53>
<cfelse>
<cfset WeekNumber=52>
</cfif>
</cfif>
</cfcase>
<cfcase value="6">
<cfset WeekNumber=WeekNumber-1>
<cfif WeekNumber IS 0>
<cfset WeekNumber=52>
</cfif>
</cfcase>
<cfdefaultcase>
<cfif WeekNumber IS 53>
<cfset WeekNumber=1>
</cfif>
</cfdefaultcase>
</cfswitch>
<cfset caller.WeekNumber=variables.Weeknumber> <webguru>iqof188</webguru>