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, and the seller can make plans for buying his or her next home. 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 out, you may want to allow a little flexibility. Otherwise, if your purchase closes a few days late you could find yourself staying in a motel with your belongings packed in a moving van somewhere while you pay storage costs.
There are also times when closing can be delayed by weeks, through no fault of your own. Have back-up plans prepared for such a contingency.