Why are weekdays in all types of Calendars in sync?

 

Have you wondered why weekdays in various Calendars are always in sync. For example:

Gregorian: says today is 10 July 2025 (Thursday).

Bengali: says today is ২৬ আষাঢ় ১৪৩২ (বৃহস্পতিবার). That would be 26 Ashadha, 1432 (Brahaspatibar, or, Thursday)

Vikram Samvat: might say Ashadha Krishna 11, 2082 VS (also Thursday).

Why would it be like that? The first day of the Calendars started on different dates: the day, month, years are out of sync, yet the weekdays are always in snyc. For example, why would it not be like this (say)?

Gregorian calendar says 10 July 2025 is Thursday
But Vikram Samvat says Ashadha Krishna 11, 2082 is a Wednesday

It is because the 7-day week is not linked to any astronomical feature (like a lunar month or solar year) but through consensus. The only thing keeping Wednesday as Wednesday everywhere is a global agreement, a human rhythm we’ve refused to let break.

It's purely a human convention, not a law of nature.

• The 7-day week has no astronomical basis like the day (Earth’s rotation), month (Moon’s orbit), or year (Earth’s revolution).
• The only reason Gregorian Thursday, Vikram Somvār, or Bangla Brihaspativar fall on the same day is because:
• Once the 7-day cycle was adopted in each system, it was never interrupted.
• Calendars were mapped carefully to real-world dates, not just floated freely.

Had any culture decided to reset or insert a weekday somewhere in history—just once—their system would have gone out of sync forever.

The shared weekdays across calendars are a historical accident of continued consensus, not a necessity of nature.

However, there were times when societies tried to break away from the seven-day week. Let's look at real historical attempts where societies tried to break away from the 7-day week—and what happened when they did.

French Revolutionary Calendar (1793–1805)

What Changed?

• Introduced after the French Revolution to de-Christianize timekeeping.
• Year divided into 12 months of 30 days each.
• Each month divided into 3 “décades” (10-day weeks), with days named:
• Primidi, Duodi, ..., Décadi
• The 5 or 6 leftover days were year-end festivals.

Consequences:

• The traditional 7-day rhythm was disrupted.
• Workers lost their weekly rest day (Sunday).
• Productivity and morale plummeted.
• People resisted, especially in rural and religious communities.

Outcome:

• Napoleon abolished it in 1805.
• France reverted to the Gregorian calendar and 7-day week.

Soviet Union’s “Continuous Work Week” (1929–1940)

What Changed?

• Introduced to maximize industrial output and undermine religious observance.
• Replaced the 7-day week with:
• 5-day week (nepreryvka, “continuous work week”) in 1929.
• Later modified to a 6-day week in 1931.
• Workers were assigned different days off, so factories never stopped.

Consequences:

• Families and friends often had different rest days—social life broke down.
• Machinery maintenance suffered due to no universal rest day.
• Religion (especially Orthodox Christianity) was targeted, but resistance grew.

Outcome:

• By 1940, the 7-day week was reinstated.
• The Soviet experiment with time had failed.

The Sabbath tradition, and the later dominance of Christianity, played a pivotal role in globalizing and entrenching the 7-day week.

Just think about it: Even when 11 days were gobbled up from the Julian Calendar to make it into the Gregorian Calendar, the weekday sequence was retained:

The old Julian calendar, established by Julius Caesar in 45 BCE, had too many leap years—it drifted by about 11 minutes per year. Over centuries, this drift misaligned the calendar with the equinox, and hence with Church festivals like Easter.

The Reform:

• In 1582, Pope Gregory XIII introduced the Gregorian calendar to fix this.
• The adjustment:

Thursday, October 4, 1582, was followed by
Friday, October 15, 1582
(10 days were “gobbled up”)

But the 7-Day Week Was Not Broken

• Importantly, the day of the week sequence was preserved.
• October 4, 1582: Thursday
• October 15, 1582: Friday

Even though 10 calendar dates were removed, the weekly cycle was uninterrupted.

The Church and civic authorities made a conscious choice to keep the 7-day rhythm intact.

England’s Turn (Much Later)

• England (and its colonies) didn’t adopt the Gregorian calendar until 1752.
• By then, the drift required skipping 11 days instead of 10.

Wednesday, September 2, 1752 was followed by Thursday, September 14, 1752

Again, Wednesday → Thursday: the weekday sequence continued unbroken.

And that, my friends, is the reason why the weekdays of all Calendars are in sync.

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