Ms. Godiva :)
"On or Before" should pretty much be the closing date. Unless you and the seller are ready and agree then it could be before that date. Have you received some information that leads you to believe that the closing date may not happen? If not, you should be closing on the agreed upon date.
"On or about" is common language in a contract regarding the closing date also. Things can happen, your mortgage can be late, there can be title issues and so on. That statement is there for the benefit of both parties. If you have a set closing date it will probably, most likely, 99.9% sure to be the closing date.. But there is always that .10% chance of something unforeseen happening.
"Time of the essence" cannot be served until the date you had written as the closing date has past... then they have 10 days. Hang on and keep the faith.
Orange Key Realty