The Sol Month: The 13th Month of the 13 Month Calendar
Everything about Sol — the new month between June and July
What is the Sol month?
Sol is the 13th month of the 13 month calendar — the brand-new month that doesn't exist in the Gregorian system. Sol sits between June and July, has exactly 28 days like every other month in the 13 month calendar, starts on Sunday, and ends on Saturday. The name comes from the Latin word for sun, chosen because Sol falls right around the summer solstice in the northern hemisphere.
When Moses Cotsworth designed the 13 month calendar in 1902, he needed a name for the new month that would feel familiar and astronomically meaningful. Sol fit perfectly: short, easy to pronounce in most languages, and tied to the sun, which mirrors the calendar's solar (not lunar) basis.
Where Sol sits in the 13 month calendar
The 13 month calendar keeps the familiar Gregorian month names and adds Sol in the middle of the year. The full order is:
- January (28 days)
- February (28 days)
- March (28 days)
- April (28 days)
- May (28 days)
- June (28 days) — followed by Leap Day in leap years
- Sol (28 days) — the new 13th month
- July (28 days)
- August (28 days)
- September (28 days)
- October (28 days)
- November (28 days)
- December (28 days) — followed by Year Day every year
Sol 1 falls on what Gregorian-calendar users call roughly June 17–18, and Sol 28 falls around July 14–15. The exact mapping depends on whether the year is a leap year and is handled automatically by the 13 month calendar date converter.
Why Sol matters for the 13 month calendar
Sol is what makes the 13 month calendar work. Without a 13th month, you can't divide a 364-day year into equal 28-day blocks. With Sol, the year breaks cleanly into 13 × 28 = 364 days, with Year Day (and Leap Day) handling the remainder outside the weekly cycle.
Sol also gives the year a true center point. Sol 14 falls on or near the summer solstice — the longest day of the year in the northern hemisphere — and the 13 month calendar puts this astronomical moment at the calendar's literal midpoint. Months 1–6 sit before Sol, months 7–12 (renamed July–December) sit after.
Sol holidays and traditions
Because Sol is a new month, there are no inherited holidays — but calendar reform advocates have proposed several. Sol 1 is sometimes called "Solstice Day," Sol 14 is often suggested as a midyear celebration, and Sol 28 marks the transition into the second half of the year. Some 13 month calendar users move existing summer holidays (like the 4th of July) into Sol to preserve their seasonal feel.
Find your Sol birthday
Anyone born between roughly June 17 and July 15 has a birthday in Sol on the 13 month calendar. Use the 13 month calendar birthday finder to see if your birthday lands in Sol — and which perpetual weekday it falls on, every year, forever.