It is absolutely essential that you include a closing date as part of your offer.  This way both you and the seller can make plans for moving.  Though most transactions actually do close on the right date, do not be so inflexible that a delay creates insurmountable problems.

For example, if you are renting and need to give the landlord notice that you are moving, you may want to allow a little flexibility. Otherwise you could find yourself staying in a motel with your belongings packed in a moving van somewhere while you pay storage costs.

Some buyers believe that the closing date is also the occupancy date.  This is not necessarily so.  Usually occupancy occurs on the funding date or the day the deed is recorded at the courthouse.  Sometimes buyer and seller sign all documents, but the lender is a day late in funding.  Thus the title company is a day late in recording and you are a day later in occupancy.  Usually these things can be addressed prior to the day of closing.