A selection of XSL Templates which I have created for both SharePoint 07 & 2010
###Using Day Format:
You will need to add this to the ContentQueryMain.xsl and SummaryLinks.xsl before being able to display this in your Item Style.
Item Style example:
Initially create Month_Short as a variable with Modified date as below (this will make a variable based on the Date modified of the item)
<xsl:variable name="Month_Short">
<xsl:value-of select="substring(@Modified,6,2)"/>
</xsl:variable>
The below will create a variable based on the above variable returning the month code 12 as 'Dec'
<xsl:variable name="Month">
<xsl:call-template name="OuterTemplate.cfShortMonth">
<xsl:with-param name="MonthShort" select="$Month_Short"/>
</xsl:call-template>
</xsl:variable>