(The reference to the "mandatum" comes in reference to the first word of the first antiphon of this ceremony: Mandatum novum do vobis... - A new commandment I give unto you...)
In the Roman rite today, the mandatum typically occurs within the context of the Holy Thursday Mass, following the homily -- though it may also occur (at least in the usus antiquior) outside of the Mass in a separate ceremony. White vestments are used for this ceremony in the Roman rite.
In the pre-conciliar Ambrosian rite, the mandatum is not done within the context of the Mass, but was instead kept as a separate ceremony. This is the ceremony that is seen in its beginnings here:
Cardinal Schuster is pictured here preaching a short homily at the beginning of the ceremony. You will note that he is not wearing vestments, but is rather wearing cappa magna and is flanked by two Milanese canons in their own cappa -- also wearing the pectoral cross; pontificals which (amongst others) the canons of the Milan were privileged to wear.
In the front row are those twelve men whose feet will be washed by the Cardinal Archbishop in the ceremony. Each of these will receive from the Cardinal Archbishop's hands a velvet bag with a small gift inside. Eleven of the men receive a white velvet bag, and one -- chosen by lot -- will receive a violet one which symbolizes Judas' burse.
Quite interesting indeed. Thanks to Nicola for sharing his wealth of knowledge of the Ambrosian tradition.
