The term "Old Style calendar" refers to the Julian calendar, which was used in various parts of the world until it was replaced by the Gregorian calendar in the 16th century. The word "Old" is pronounced /oʊld/ with a long "o" sound and "d" pronounced as /d/. "Style" is pronounced as /staɪl/ with a long "a" sound and "l" pronounced as /l/. The spelling of this word follows English phonetic rules and is reflective of its historical origins.
The Old Style calendar, also known as the Julian calendar, is a historical calendar system that was used before the adoption of the Gregorian calendar in many parts of the world. It was introduced by Julius Caesar in 45 B.C and was in use until the late 16th century in Western Europe, and even later in certain regions.
The calendar was based on a solar year of 365.25 days, with a leap year every four years to accommodate the extra quarter day. This method of calculating leap years, however, resulted in a slight overestimation of the solar year, causing the Julian calendar to gradually fall out of sync with the solar year over time.
As a result, the Julian calendar had to be adjusted periodically to maintain its accuracy. In 1582, Pope Gregory XIII introduced the Gregorian calendar, which made a more accurate correction to the Julian calendar's overestimation of the solar year. Many countries subsequently adopted the Gregorian calendar, which is still in use today in most parts of the world.
Nevertheless, some countries, particularly Eastern Orthodox nations, continue to observe religious holidays and events based on the Old Style calendar, resulting in a discrepancy between their dates and those followed by most of the world.