There is a wealth of official published material detailing the major places, persons, and events of the Flanaess. The published, or canon, material starts in the Common Year (CY) 576, and goes through CY 591. In those 15 years, the setting undergoes enormous changes both in detail and in tone.
The Dungeon Master must answer two fundamental questions in regards to his Greyhawk campaign prior to play. First, what year will the campaign being? Second, what impact (if any) will the larger events in the Flanaess have on the PCs (and vice versa)? Of course, for DMs who are relatively unconcerned with canon or setting, these questions are irrelevant, but it does raise the question of why use the World of Greyhawk in the first place.
The three most popular time-periods for starting play are CY 576, 585, and 591. Each has strengths and weaknesses, and each will give a different play experience, as the tenor and tone of the setting changes dramatically as the fortunes of good and evil wax and wane.
This is the period detailed in the original World of Greyhawk folio published in 1980 and “gold box” set published in 1983. Evil is on the rise in places like Iuz and the Sheldomar Valley (encompassing Keoland and the states surrounding it), but there seems to be a status quo in place and there are bright places of hope willing to stand against it such as Veluna, Furyondy, Nyrond, and the Iron League. The tone is one of good and evil in balance, with the player characters able either to tilt the balance in the direction of good in their way, or to pursue their own ends without seeming to leave the rest of the world to its fate.
This is the time of the Greyhawk Wars (1991) and From the Ashes (1992) boxed sets. By this time the Flanaess has undergone enormous turmoil, and the tide has turned most decidedly in favor of chaos and evil. Iuz has conquered the Horned Society and the Bandit Kingdoms, and is putting serious pressure on Veluna and Furyondy the Vesve Forest is a battleground. Almor is ruined, Nyrond is exhausted, and the Great Kingdom itself has fallen into anarchy, ruled by a series of undead, demonic, or otherwise evil-serving warlords. Geoff and Sterich are either lost (or soon will be) to giants and humanoids. The Circle of Eight, those powerful mortals who strove to maintain balance, are gone. The Scarlet Brotherhood has created an empire in the south seemingly overnight, conquering the Sea Princes and establishing outposts throughout the Flanaess, threatening or overturning several members of the Iron League. The tone of the setting is one of evil ascendant, with the player characters there to slow the tide of evil. Reversing it hardly seems possible.
This is the period described in the Greyhawk Player’s Guide (1998), The Adventure Begins (1998), and the Living Greyhawk Gazetteer (2000). Some semblance of balance has returned to the Flanaess, but seemingly only because both sides are so exhausted from the fighting that they are hunkering down and rebuilding. Some successor states have arisen in the former Great Kingdom, the demons and other denizens of the lower planes which had been running rampant across the face of the Flanaess were mostly sent back to their home planes. Furyondy and Veluna have dealt checks to Iuz. Sterich has been retaken, and Geoff is not as solidly lost as was once supposed. The Scarlet Brotherhood has been set back on its heels, and Nyrond, though wounded, is regaining its strength. The setting’s tone at this juncture in history is seemingly one of holding its breath, waiting to see if the pendulum will swing in the direction of order and good, or chaos and evil. The time is ripe for player characters to intervene and influence the swing of that pendulum.
Naturally, these are only the time periods that have been extensively detailed in published products. It’s entirely possible for a DM to set a campaign a hundred years before or after these events, allowing him to completely remake the tableau of the Flanaess and chart his own course.
However, for the DM who is desirous of taking advantage of the rich history that has been created, there are two ways to go.
The first is to give the player characters the opportunity to be among the movers and shakers in the setting. There are three ways to go about this. They could themselves be nobles, commanders of armies, heads of guilds, etc. and influence the course of events on a grand scale. Or, they could be relatively anonymous adventurers, acting at the behest of those grander individuals, helping to stem the tide of evil. Or, as a third possibility, they could simply stumble on opportunities to influence the course of history, simply being in the right place at the right time, and having no more motive than the desire to Do the Right Thing (and get well remunerated in the process!).
The second alternative is to have the well-documented events happening in the background, with no direct input or influence from the player characters (an approach I call The Great Greyhawk Campaign). This has the advantage of not being dependent on the actions of the player characters, allowing the Big Events of the campaign tableau happening in the background (and still allowing the PCs to act on the periphery of those events), but the timeline won’t be thrown out of whack if the PCs somehow stop Iuz from wiping out the Horned Society in CY583.