When you write a book, you are inviting your reader to enter a world of your own creation and asking them to believe that what you're telling them is either real or could really happen. You create this atmosphere through your choice of words and rely on the reader's imagination to provide the picture. You and the reader work as a team to form the entire scope from what the city/town/planet looks like to how the characters' voices sound.
Within the first page or two of the book, your reader needs to know where they are so they can start creating this fictional world in their minds. If they don't know they're supposed to be in an office and ten pages into the story, a character turns around and puts a document in the Xerox machine, and the reader was imagining a forest, it will throw them completely out of the loop and make them feel unsure about their footing. If your story is taking place in the past, make sure your reader knows this immediately.
Bad Example #1: Yasmine sighed as she looked at the calender that hung on her brown paneled office wall, thirty stories above the Manhattan shopping district, and noted that it was January 18th, 2006.
We can see through this example where she is (in her office in Manhattan) why she's there (she works there) and what day it is (1/18/2006) But do we care? I don't know about you, but I don't, and I wrote the thing! If anyone should care, it's me!
Sometimes the simplest way to establish a date, especially if you're writing historical fiction, is to include the date in the chapter heading. Otherwise, you're stuck doing stuff like:
Bad Example #2: George couldn't believe it had been a whole year since the assasination of Martin Luther King.
Okay, this gives us a time frame, but if Martin Luther King doesn't immediately play into the story or into George's life, don't use this as a time frame just to get the reader to know the date. It's a stiff ploy.
So here's a good rule of thumb: Take a look at the first two pages of your manuscript. Who is your character, where are they, and why are they there? Nichole Giles over on LDSWritersBlogck wrote a great blog on this topic called "Giving it Away." http://ldswritersblogck.blogspot.com/2006_07_09_ldswritersblogck_archive.html
In summary, if your reader doesn't know where they are in your story almost instantly, you need to rewrite. Imagine if you had been blindfolded, shoved in a van, driven around for an hour and then dumped in a remote location, not knowing one thing about your new surroundings. Don't do this to your reader. Provide their surroundings immediately so they can start to feel at home in your story.