The World Calendar is a 12-month, perennial calendar with equal quarters. It is perennial because it remains the same every year.
Our present Gregorian calendar is not perennial, but annual. It changes every year. It does so because its typical 365-day length, which represents the annual cycle of one Earth orbit around the Sun, is not evenly divisible by the number of days in the week (365 / 7 = 52, r 1). The unfortunate consequence of that one-day remainder is that the year typically begins and ends on the same weekday. So the next year must begin on the following weekday. This requires a new calendar every year.

Technically, our Gregorian calendar is a variously ordered cycle of 14 calendars. The calendar for the year beginning on Sunday differs from the one for the year beginning on Monday, and so on for all seven weekdays. Since the occurrence of leap year can alter any of these seven calendars, this raises the total to 14 calendars.
That's the mess the 365th day causes. If we could take that day out of the calendar, the new year would typically begin on the very same weekday as the previous year. And if we likewise could take leap day out of the calendar, the new year would always begin on the same weekday. We'd thus have a perennial calendar.

That's the rationale on which The World Calendar is based. Without deviating from the solar cycle of approximately 365.24 days, it simply regards the 365th day as a 24-hour waiting period before resuming the calendar again. These off-calendar days, also known as "intercalary days," aren't weekdays. It seems most reasonable to treat them as holidays.
More Benefits of The World Calendar

The World Calendar, promoted by The World Calendar Association beginning in 1930, has noticeable benefits for scheduling and planning that the Gregorian calendar lacks. Other advantages are less obvious.
Numbered days of the month always fall on the same weekdays: so the birthday of Tuesday's Child is always on a Tuesday.
No need to schedule events by cumbersome weekday-and-month designations, like "The first Tuesday after the first Monday in November".

The year divides regularly into quarters of equal size (91 days), with the same number of workdays (65) and weekend-days (26) in each quarter: a great improvement over the Gregorian calendar for statistical comparisons between quarters.
The variations in month-length are more regular than the Gregorian calendar: most months have 30 days; the first months of thequarters (January, April, July, October) have 31.
Excluding Sundays, all months have the same number of days: 26.

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"Write Dates with Numerals In Order!" Thomas Says