Generally in the English speaking world the calendar of 365.242 days is used. This means that every 4 years a leap year is created – basically the 0.242s add up to a whole day and we have to put it somewhere. So February, already short in days, is given the extra day: February 29.
The exception to this rule is years that are divisible by 100, with another exception to this rule being years divisible by 400; they are leap years. Don’t you love simple exceptions! There are 52 weeks each year plus one day. I am sure you know that this hasn’t always been the case. Not that long ago a different calendar was used, the Julian Calendar, and though it was close to perfect, eventually things went wrong. 10 days wrong to be exact.
Pope Gregory XIII finally decided not to use Julian Calendar (names after Julius Caesar and started in 45 BC) and go for a more scientifically correct one that we still use today. Before that, the Roman Calendar was used; it seems a little complicated to us as the months had Nones, Ides and Kalends around which one counted the days. These months were based on the phases of the moon with the Ides being roughly when the moon as full. In some countries the lunar calendar is used as the official calendar rather than the Gregorian Calendar used in most countries.
Did you know that the oldest lunar calendar was actually found in Scotland? It dates back to 8000 BC.
If your students are studying economics then they will know about a fiscal calendar. Fiscal years do not always correspond to the Gregorian Calendar. All of your students will know about the School Calendar, though they might call it by other names. This is the year of when they attend school, and more importantly for some of them, when the school is on holiday. You might ask them to find out if the whole country has the same ‘school’ (or academic) year, if there are differences between primary, secondary and tertiary schools and institutions, or even if in the city or town where the live schools all follow the same calendar.