The coverings should not be put up until Saturday 28 March 2009.
The Roman Missal’s rubrics for Saturday of the fourth week of Lent have:
“The practice of covering crosses and images in the church may be observed, if the episcopal conference decides. The crosses are to be covered until the end of the celebration of the Lord’s passion on Good Friday. Image are to remain covered until the beginning of the Easter vigil.”
(Roman Missal, Catholic Book Publishing Co., New York, 1985, page 113.)
There does not seem to be any requirement that the coverings be purple.
The vestment colours are as direction in the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from romanrite.com/girm.html :
“346. As to the color of sacred vestments, the traditional usage is to be retained: namely, …
d. Violet or purple is used in Advent and of Lent. It may also be worn in Offices and
Masses for the Dead (cf. below). …
f. Rose may be used, where it is the practice, on Gaudete Sunday (Third Sunday of
Advent) and on Laetare Sunday (Fourth Sunday of Lent). …”