There are times when the "for sale" sign remains, even when there is a contract pending. If the pending contract carries a sale of home contingency (meaning the buyer must first sell their house, before buying this one) there is often a clause that allows the seller the right to continue marketing the property, and seek additional offers.
Each situation is different, but once you have actually closed the deal, and the property is officially yours, you are of course free to remove the sign and lockbox.
Have a Merry Christmas, and enjoy the New Year in your new home!
I just want to touch on the lockbox issue ... oftentimes the lockbox will remain on the property up to the settlement date and for good reason. You and your agent will need access to the property for the home inspection and for the pre-settlement walk-through (which usually occurs the day before or day of). This is a pretty common practice. No worries, once the property goes 'pended' status (all conditions/contigencies have been met), the property is yours, unless you default.