The 31st
Overview
The 31st is the 31st day, the maximum number of days a month can have in the Gregorian (solar) calendar. Of the 12 months, seven—January, March, May, July, August, October, and December—have a 31st, and December 31 is the last day of the 365-day common year and the boundary to the next year. Various systems, including calendar frameworks, contract periods, and settlement cycles, are organized around the number '31,' giving it social significance beyond a simple date.
Main details
Months with and without a 31st
- Months with a 31st (long months, 大月): January, March, May, July, August, October, December — seven months in total
- Months with 30 days (short months, 小月): April, June, September, November — four months in total
- February: 28 days in common years, 29 in leap years
Calculating the ordinal day of each 31st in the year gives January 31 as the 31st day, March 31 as the 90th, May 31 as the 151st, July 31 as the 212th, August 31 as the 243rd, October 31 as the 304th, and December 31 as the 365th (in a common year).
The 31st in calendar history
The maximum month length of 31 days is the result of the allocation of days per month originating in the Roman calendar and continuing through the Julian and Gregorian calendars. A common story holds that Augustus took a day from February to make August, the month named after him, 31 days long, but this is closer to a later anecdote; the actual adjustments to monthly lengths were formed in complex ways through the religious and administrative needs of Republican Rome and the operation of intercalary months. When the Gregorian calendar was introduced in 1582, leap-year rules became more precise, but the basic distribution of 31-day and 30-day months was retained.
The nonexistent 31st: February 31
Because February has no 31st, 'February 31' is used as an idiomatic expression meaning a fictitious date. Historically, there have been exceptions that actually existed during calendar reforms. In the process of transitioning to the Gregorian calendar, Sweden inserted February 30 in 1712, a case often cited as an example of the confusion between the Julian and Gregorian calendars. In programming as well, whether a date library treats an input of 'February 31' as an error or rolls it over to the next month is an important test item.
Cultural and social significance
- October 31: Halloween, one of the largest informal festival days in the Western world.
- December 31: The day of year-end and New Year's Eve (送舊迎新). In Korea, the year's last sunset and first sunrise events, bell-ringing ceremonies, year-end award shows, and concerts are concentrated on this day, and it coincides with the end of the fiscal and academic years.
- Month-end settlement and payments: Because card bills, telecommunications fees, subscriptions, rent, and similar charges are often billed on a 'month-end' basis, the 31st becomes a day when personal and corporate cash flows converge.
- Wages and allowances: In workplaces with 'month-end payment' rules, paydays differ between months with a 31st and months without one, which can become an issue in labor management.
The 31st in law and administration
In civil period calculations, the first day is in principle not counted, so the expiration date of a 'one-month' period varies between the 30th and 31st depending on the corresponding date in the relevant month. If a contract states 'the last day of each month,' the expiration date is the 31st in months that have one and the 30th in months that do not (February 28 or 29), and disputes over this difference are not uncommon. For this reason, practice recommends specifying a concrete date instead of 'the last day.'
Recent trends
As of 2024–2025, changes related to the 31st fall broadly into three streams. First, discussions about designating December 31 as a public holiday have been raised repeatedly. In Korea, December 31 is not a statutory public holiday, and criticism continues that year-end work and settlement burdens are concentrated on that date; the government has considered designating it as a temporary public holiday as needed. Second, with the spread of the subscription economy, 'month-end billing' has become central to personal consumption structures, and consumer inquiries about differences in billed amounts between months with a 31st and months without one have increased. Third, with digital transformation, automation of month-end closing has spread, and a standard pattern has taken hold in which accounting and logistics systems perform batch work at midnight on the 31st. Meanwhile, December 31, 2025 falls on a Wednesday, and as it is a period when year-end demand and travel demand overlap, local governments separately establish safety management plans.
Related topics
- [[Gregorian calendar]]
- [[February 29]]
- [[Halloween]]
- [[December 31]]