[..snip.]
<xsl:variable name="veranstaltungen" select="//news/month" />
<xsl:when test="$veranstaltungen[@year=../@date]/@name =
substring (@name,1,3)">
    do something
</xsl:when>
[..snip.]

Why does the "../@date" in the while clause not contain (at
least it seems so) anything?

You haven't shown us your XML source, but my guess is that you are imagining
that ../@date selects the same thing inside the predicate as it would select
outside the predicate. In fact the context node changes inside the
predicate. Perhaps you want

test="$veranstaltungen[@year=current()/../@date]

Michael Kay
http://www.saxonica.com/