The link is working for me.
The important details are:
To take advantage of this special White Christmas deal, use code “WHITEXMAS” and place your order before midnight Pacific Time on December 31st.
Here is the fine print:
The Fine Print: This promotion is non-transferable and valid 2x per customer on eligible products totaling up to $2,500 ($500 maximum discount per order). This promotion may not be combined with other discounts or promotion codes in one order. 20% discount applies only to 3D prints of your own designs in White Strong & Flexible and White Strong & Flexible Polished and does NOT include dyed Strong & Flexible prints. Promotion codes cannot be applied to Shapeways Gift Cards or past orders. If you order a model during the promotion period that cannot be printed and has not been successfully printed before, we cannot apply discounts to future orders (even if these models are repaired). If you return your purchase, you will be refunded the amount paid. Offer expires December 31, 2013 at midnight Pacific Time.
Bold emphasis added by me.