One of the challenges in creating and running a campaign is keeping track of dates. To do this, you need to create a calendar that fits in with your campaign. Not every world will have a 24-hour day or 365 days in a year but it is important to make those choices relatively early in your campaign. Not only does a calendar help track any seasonal changes (if any) but it will assist you in creating your living world.
What use is a calendar?
The calendar assists the game master in a number of important ways. It encourages proactive thinking about your campaign setting. Are there holy days that need to be accounted for? Are there civic (or other) events that occur on a regular day? Is there a common day of rest – religious or otherwise? Are there “truce days” when fighting is forbidden?
This also helps with the task of character ageing, as pointed out in the Gnome Stew article “Character Birthdays and Advanced Ageing”  by J.T. Evans, because it allows the game master (or the players) to choose a “birthday” for the characters. Building a calendar can also be a constructive “Session Zero” activity.
A calendar can help establish some fundamental guidance to provide context for your campaign setting. For example, the old Roman or Julian calendar (on which our current calendar is generally-based) had market days set every nine days and certain days each month were considered lucky or unlucky (but not Friday the 13th). The ancient Greeks divided the year into 10 months. There may be different calendars by race (Tolkien wrote that the Elven “long year”, for example, is 144 human years), religion or region.
If you are running a space-based campaign, where there are worlds with different daily rotations and different orbits around their star, you might have to consider the local calendar as well as the “Interstellar Standard” (if any).
Customize your Calendar
You can also decide the number of days in a week or month, or dispense with the idea of weeks and months entirely.
In ancient Rome, the numbering of years started with the founding of the city 753 years BCE. In mediaeval times, the years of reign by a sitting monarch (the third year of Kind Edward, for example) was used to date edicts, contracts and so on, which made calculating the overall passage of years difficult.
It may be that, as in George R. R. Martin’s Song of Fire and Ice, your world has irregular seasonal changes. An annual calendar is still required, if only to regulate the collection of taxes, the dating of contracts and marking the passage of time.
Calendars reflect your living world
If your world does have seasonal changes, a calendar assists the Game Master in how the seasons work and the impact of these seasons on daily weather. As in parts of our world, there may be only two seasons (rainy season and dry season), or different seasonal variations, such as a three-season Mediterranean/California-type climate. All of these would have a different impact on daily weather (or your daily weather control for advanced sci-fi worlds).
Thinking about the calendar helps the Game Master in the act of world-creation. It also assists in creating a more immersive world for the player characters. Do the characters need to prepare for winter? How does the weather impact the way characters dress? Plate mail, for example, would be extremely uncomfortable in either high summer or deep winter. Would three days of rain affect the ability of massed troops to move? Would drought have implications for your adventuring party?
In any case, creating a calendar for your campaign (or world) is a useful tool. You need not have everything nailed down at the start but it will assist both the Game Master and the players in doing their forward-planning. Make a date (or dates) today!