Expired domains have a grace period to give the owner a chance to register a domain that has run out. This is common practice in the industry to stop people hijacking or drop catching expired domains.
http://en.m.wikipedia.org/wiki/Domain_drop_catching
I've been told that squarespace doesn't allow use of the grace period, to register an expired domain.
My domain was registered with squarespace, I closed the site, the domain expired and now its in the grace period, where its owned by squarespace, they wont let me reregister it, and at the end of the 40 days grace period both squarespace and Tucows (their domain provider) policies say the domain may be auctioned or incur a $175 reregister fee.