Calendars make the life of a DM that much easier, and it is a way that we can save our work! You can more accurately control the game, it gives you specifics at a glance for information that you’ve chosen to record. If a magic item can only be used 3 times a week, there is no excuse for you not to keep this rule in check now. You can get as elaborate, or as simple as you want to get with them, as long as you find a system that works for you, and as a bonus, YOU GET SEASONS!!!! Fighting through raging blizzards! Feeling the relief of a long awaited and much needed rain. Celebrating their gods through ancient rites. Knowing when to construct a Sword which honors the God of Justice, in a way that, if he sees fit, he may bless with his divine power.
Again, it's exactly the sort of article we used to see in magazines like Dragon back in the day. (And seriously, Ripper X, next time you put that sort of work into something, you might consider selling it to a magazine like KoDT or somebody. If you're going to work that hard, there's no shame in getting paid for it. ;) )
Ripper X gives a great example of a Viking-influenced calendar based on the motions of the moon. For your edification, here's a solar calendar for a tropical culture. This one was built around a zodiac inspired by the traditional four elements, and incorporates a system of virtues and vices as well as your typical seasonal influences.
The Zodiac used in the world of Mitihum was created by the Kesh. All others have adopted it from them, and it spread throughout the world. It is very popular among sorcerers who consider children born under EAGLE to be especially blessed.
Earth1 – TURTLE: Foundation, stable/calcified; opposite is Fire2
Earth2 – CAT: Earth Mother, fruitful/capricious; opposite is Air2
Earth3 – ELEPHANT: Wealth, productive/avaricious or hoarding; opposite is Water1
Water1 – PELICAN: Rain, nourishing/drowning; opposite is Earth3
Water2 – VEIL: Ocean/Keeper of Hidden Wisdom, intuitive/irrational; opposite is Air1
Water3 – SERPENT: River/Maker of Ways, directed motion/obsession; opposite is Fire3
Fire1 – DRAGON: Scourge, purifier/destroyer; Opposite is Air3
Fire2 – RAVEN: Liberator, freedom/anarchy; opposite is Earth1
Fire3 – CROWN OF WONDERS: Halo/Crown of Wonders, imagination/distraction; opposite is Water3
Air1 – EAGLE: Reason, clarity/detachment; Opposite is Water2
Air3 – JEWELED NECKLACE: Mercy, healing/indulgent; opposite is Fire1
Ok, so now we need to arrange these into months and seasons. First, what is the weather like?
Spring Equinox – Beginning of new year.
Spring First: Jeweled Necklace – rainy, cool. 29 days.
Spring Second: Cat – flowers and fruits after previous rains. 32 days.
Third Spring: Pelican – if the rains are mild, this is a very fruitful time. However, excessive rains can cause mudslides, flooding, and other destruction. 29 days.
Summer First: Elephant – time for fruit harvests, shipping to the mainland. 32 days.
Summer Second: Serpent – traditionally a time when the young are inducted into adult society, also a popular time for weddings. 29 days.
Summer Third: Raven – The heat of summer is wearing on, fruit and stores must be eaten now or prepared for long storage, festivals end the month, followed by a period of fasting. 36 days.
Autumn Equinox – winds shift
Autumn First: Dragon – Shifting weather patterns causes chaos at sea, wild storms. Also the beginning of the raiding season, and a period of fasting and introspection, cleaning and repairing. 29 days.
Autumn Second: Eagle – A time of intellectual endeavor and learning. Much shipping from the mainland, including grains and cattle. 32 days.
Autumn Third: Veil – As nights grow longer, traditional period of plays, storytelling, politics and assassination, and other secrets. 29 days.
Winter First: Turtle – The sun begins to return. Tools are repaired, fields are cleared in preparation for planting. 32 days.
Winter Third: Crown of Wonders: Preparation for the oncoming monsoons, repair of homes, shoring up waterways and bridges. 32 days.
A normal year has 370 days in it. Every three years, Raven loses one of its days to keep the equinoxes and solstices from drifting out of their proper months.