Change <xsl:value-of select="periode[@aankomst]"/>
to <xsl:value-of select="periode/@aankomst"/>
In XPath, read "a[b]" as "select a where b is true". So you are selecting periode if there is an @aankomst, and the value of the periode element is gast 3.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference