In the Julian calendar the relationship between the days of the week and the dates of the year is repeated in cycles of 28 years. In the Gregorian calendar this is still true for periods that do not cross years that are divisible by 100 but not by 400. A period of 28 years is called a Solar Cycle . Let us further confine ourselves to the years 1901-2099. So you can always reuse your calendar every 28 years.
Different countries have different customs.
This confusion leads to misunderstandings. If you want to be sure that people understand you, I recommend that you write the month with letters instead of numbers, and write the years as 4-digit numbers. Jesus was born under the reign of king Herod the Great, who died in 750 AUC, which means that Jesus could have been born no later than that year. Dionysius' calculations were disputed at a very early stage. Although Bede seems to have used the term on at least one occasion, it is generally believed that B.C. dates were not used until the middle of the 17th century.