position() is always based on the nodes position within the currently
selected node list, not its position within the XML document.

I'm not sure why, but this seems to be a common misunderstanding about
position(). position() has _no_ relationship to the position of a node
in a source document, it just relates to the position of a node in the
current node list (in xslt 1) or the current item in the current
sequence (in xslt2).

No, it returns the position in whatever set of nodes you are currently
processing. Those might be siblings, or they might not.

I'm not sure what number you want, probably <xsl:number/> or count(preceding-sibling::*)+1

http://www.dpawson.co.uk/xsl/sect2/N6099.html#d8191e311

So to get the result you're after you need to build a node list and
then get the position of the node you're after:

<xsl:attribute name="daynr4">
 <xsl:variable name="genID" select="generate-id(parent::*)"/>
 <xsl:for-each select="../../day">
         ^^^^^^^^^^^^^^ This builds the node list
   <xsl:if test="generate-id() = $genID">
     <xsl:value-of select="position()"/>
   </xsl:if>
 </xsl:for-each>
</xsl:attribute>

The above is of course a waste of time, you should just use 1 +
count(parent::*/preceding-sibling::*))

Kind regards,
-- Abel Braaksma
  http://www.nuntia.nl