Under the doctrine of eminent domain, a U.S. federal, state, or local government has the power to take private property and convert it into public use. However, the government must first give you, the property owner, notice of its intent to take the land.
Each state's statute sets forth a different notice period and a different set of requirements that the government must follow in order to inform you of the planned taking.
For example, Massachusetts M.G.L.A. 79 § 8B notes that "No person in possession of property which has been taken... shall be required to vacate [it] until four months after notice of such taking." In Pennsylvania, 26 Pa.C.S.A. § 305 requires that the government first publicly file a lis pendens (essentially a notice of its intent to take the land, filed with the county clerk), and then notify you within 30 days of that filing.
Regardless of your state's particularities, notice of the planned taking will probably be given to you by personal service (through a process server) or by registered mail. The letter will explain the date by which the government expects to take possession of your land.
Typically, before that date, you will have an opportunity to retain a lawyer and contest the taking, if you so choose.
Even if you do not contest the taking, and instead decide to accept whatever reasonable compensation the government offers you, you would still need that time period within the notice window to clear out the land (for example, if you had personal property stored there).
Go to the main eminent domain FAQ page.