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The Thirteen Month Leap-Month Calendar is an alternative calendar that consists of thirteen months, each containing 28 days of exactly four weeks, which compose an ordinary year of 364 days, containing exactly 52 weeks. Once, after 293 months have passed, a leap-month of 28 days is added, creating a leap year of 392 days. This would occur on average every 22.53 years. Thirteen cycles of 294 months compose a full calendar cycle of 107,016 days, 15,288 weeks, and 3,822 months of exactly 28 days each, after which the cycle repeats itself.

The Epoch for this calendar would be Monday, January 1, 1 of the Proleptic Gregorian calendar. Every week, month, and year would commence a Monday. The begining date of the past, current, and upcoming cycles would be:

Cycle beginnings
Monday, 1 January 294

Leap months would occur in the 293 year cycle in the following sequence:

Leap month occurrences
Year Month Esperanto Leap Month Name
022 07 Salti-Sepa
045 01 Salti-Unua
067 08 Salti-Oka
090 02 Salti-Dua
112 09 Salti-Naua
135 03 Salti-Tria
157 10 Salti-Deka
180 04 Salti-Kvara
202 11 Salti-Dekuna
225 05 Salti-Kvina
247 12 Salti-Dekdua
270 06 Salti-Sesa
293 13 Salti-Dektria

To distinguish the months from the months of the Gregorian calendar, the months would be named after ordinal numbers in Esperanto: Unua, Dua, Tria, Kvara, Kvina, Sesa, Sepa, Oka, Naua, Deka, Dekuna, Dekdua and Dektria. The leap month, when it occurs, would be named after the prior month, with the prefix Salti- (Esperanto for leap-), such that the leap month after Deka would be called Salti-Deka. The leap month would be part of the year containing the prior month.