Saturday delivery shipments are almost identical to regular DPD Classic shipments. You only have to add a flag <saturdayDelivery> to the <productAndServiceData> section and set it to true.
Sample call
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns="http://dpd.com/common/service/types/Authentication/2.0" xmlns:ns1="http://dpd.com/common/service/types/ShipmentService/3.3">
<soapenv:Header>
<ns:authentication>
<delisId>KD*****</delisId>
<authToken>****</authToken>
<messageLanguage>en_EN</messageLanguage>
</ns:authentication>
</soapenv:Header>
<soapenv:Body>
<ns1:storeOrders>
<printOptions>
<printerLanguage>PDF</printerLanguage>
<paperFormat>A6</paperFormat>
</printOptions>
<order>
<generalShipmentData>
<sendingDepot>0530</sendingDepot>
<product>CL</product>
<sender>
<name1>Senders NV</name1>
<name2>Jan Janssens</name2>
<street>Egide Walschaertsstraat 20</street>
<country>BE</country>
<zipCode>2800</zipCode>
<city>Mechelen</city>
<type>B</type>
</sender>
<recipient>
<name1>Receivers NV</name1>
<street>Teststraat 5</street>
<country>BE</country>
<zipCode>2800</zipCode>
<city>Mechelen</city>
<type>B</type>
</recipient>
</generalShipmentData>
<parcels>
<customerReferenceNumber1>Box 1234</customerReferenceNumber1>
<weight>340</weight>
</parcels>
<productAndServiceData>
<orderType>consignment</orderType>
<saturdayDelivery>true</saturdayDelivery>
</productAndServiceData>
</order>
</ns1:storeOrders>
</soapenv:Body>
</soapenv:Envelope>