Especially at higher levels, player characters sometimes undertake plans of action that take large amounts of time to come to fruition. The basic units of time larger than the days tracked in wilderness exploration are the week, fortnight, month, quarter, and year, none of which is entirely satisfactory, although months seem to be the most salient.
Again, the first place to look is the medieval record. One nice unit of time that pops out is the Knight's Service, usually given as 40 days, but sometimes as 45, which is also the figure usually given for the military service obligation of Anglo-Saxon freemen, and, by extension, probably a reasonable estimate for many other Viking-influenced cultures. The later figure amounts to about a month-and-a-half, six-and-a-half weeks, just over three fortnights, and, most interestingly to me, one-half of a standard season or quarter.
What I find neat about the half-season is that it lines up with the neopagan wheel of the year, which amalgamates several seasonal festivals of paleo-pagan Indo-European cultures, ones that all happen at natural "focusing points" of the year. So, we could break up the year into these half-seasons, each marked by a small festival marking the beginning or middle of a season (the solstices, equinoxes, and cross-quarter days) that would very easily fit into most medieval-inspired settings very naturally. Conveniently, a single half-season "turn" would also be the amount of time that a character could draw on resources like the military service of vassals and a reasonable chunk of time to retain mercenaries or specialists. It's probably also a reasonable estimate for a how much corvee-labor a liege can get from their serfs each year.
Unlike turns and days (but something like the watches in my city-exploration draft rules), each particular half-season would have its own character and social rhythm, imposing some limitations on what characters may effectively accomplish in that time. I've listed the half-seasons along with the modern months they would correspond, example festivals, and some notes about the seasonal character:
- Early Spring: (February to mid-March) beginning with the Feast of Lights, the first day of the year and the day on which everyone adds another winter to their age, a time of planting for spring fields and gardens.
- Late Spring: (mid-March through April) starting with the Vernal Equinox, when planting is finished and armies begin to gather and march.
- Early Summer: (May to mid-June) starting with Bonfire Night, a time favored for courtship and marriage as well as battle.
- Late Summer: (mid-June through July) starting at Midsummer on the Estival Solstice, often the hungriest months of the year, prior to the first harvest.
- Early Autumn: (August to mid-September) starting with the Feast of Bread to celebrate the first fruits of last year’s winter crops: wheat or rye used to bake bread.
- Late Autumn: (mid-September through October) starting with the Harvest Festival on the Autumnal Equinox, celebrating the second harvest of spring crops and the fruits of vine and tree.
- Early Winter: (November to mid-December) starting with the Night of the Dead, heralding the death of the year and remembrance of ancestors amid preparations for the dark days of winter, along side the planting of winter wheat and rye.
- Late Winter: (mid-December through January) starting with Midwinter or Yule on the Hibernal Solstice, the cold of the year where those that are able take shelter in their homes and see to their care.
What I like about this scale is that it's a good amount of time to accomplish things like domain management activities such as construction or development, realistic periods of convalescence, whole exploratory or military campaigns, enchantment or spell research, and the like, with a full period of "rest" fitting nicely into the winter phase. It also allows parties to do things like meet "the day after midsummer" and give the players and referee a good idea of what can be accomplished in the downtime allowed, along with what resources are available to domain-owners each year.