Some Notes on the Calendar
An interesting aspect of the Forgotten Realms setting is the Calendar of Harptos. As you can see in more detail in the Sword Coast Adventurers’ Guide (SCAG), the calendar provides for a 365.25 day year, with 12 months of 30 days each. There are 5 annual feast days, and 1 quadrennial, to make up the slack. Much like the calendar we are all used to. But there are a few oddities that crop up — plot holes, as it were — that I just had to fill.
No Day Names?
The SCAG mentions that Faerunians don’t name their days. I don’t buy that, given the prevalence of religion and living gods in the Forgotten Realms. So, I propose day names given at right. I leave it as an exercise for the reader to trace their origins.
A Ten Day Week
The Calendar of Harptos has a quirky bit — a week is 10 days long, which the denizens of Toril purportedly refer to as a ten-day. Personally, I don’t buy that they call it a ten-day. Common is basically English, and we call it a week, which derives from words meaning change or alteration , rather than “a seven day period”. We used to have a word for seven days, sennight, but it has gone the way of fortnight. Actually, it’s deader, since people still use fortnight, if only occasionally. So, in my games, we use “week” to refer to the three 10-day components of the Toril month.
What in the Heavens?
The 10-day period for a week had me scratching my head. True, the ancient Chinese had a calendar with 10- and 12-day periods, but in the West the 7-day week is ancient and very likely was adopted from the quite obvious division of the lunar phases. The moon follows a roughly 28 day phase cycle , with New, First Quarter, Full, and Third Quarter falling 7ish days apart. So what in the heavens, I wondered, could have prompted the adoption of a 10-day week? What could happen every third of a lunar revolution that would be so globally obvious as to be adopted as a communal period of time?
Further reading on the Toril sky revealed that Selúne (Toril’s moon) has a group of Trojan satellites, called Selúne’s Tears. They are in her L5 Lagrange point, 60° behind her in the night sky. So it hit me. What if a large “Tear” had never gotten trapped at L5, but rather ended up in a lower orbit as a second natural satellite ? Furthermore, if this satellite — let’s call it Fengar — was close enough to Toril to orbit twice as fast as Selúne, and its orbit was retrograde, then each time Selúne went 1/3 of the way through her orbit, Fengar would go 2/3 of the way through his (in the opposite direction), and they would meet in the sky.
In other words, every ten days, Selúne and Fengar would align, and this natural periodicity would be a perfect framework for the 10-day week. Like an exuberant dog rushing away then returning to his master, Fengar would come to be known as “Selúne’s Hound”. Fengar is roughly 20% as massive as Selúne and its diameter is approximately 1/3 of Selúne’s, making its apparent size roughly half of Selúne’s apparent size. As with Earth’s Moon, Selúne’s apparent size is roughly the size of your thumb held at arm’s length.
 Actually 29.530587981 days, now (https://en.wikipedia.org/wiki/Lunar_month#Synodic_month )
 Yeeessss. Selúnes Tears are also natural satellites.