This is mostly a scratchpad to post notes on Taura's concalendars and related stuff as I'm writing again the article about it, but feel free to put stuff about your conworld's calendars as well.
The Megadelanean Calendar (also known as the Euremegadelanean, Bredezhan and Midonid Calendar) is the time reckoning system used by most peoples in Bredezhanya, among them the Hellesans. The calendar is almost as old as the amount of years it reckons, and its last iteration was established in the Council of Midònia, in 1860 MR (Megadelanean Reckoning).
Since Taura is Earth in a parallel timeline, natural years are like ours, and reckoning years tend to have 365 days in hybrid calendars (360 in some lunar calendars). The Bredezhan Calendar consists of 12 months in a 365-day year. Summer solstice marks the end of spring and the beginning of summer, but also the new year. The last day of the year is bressaures 5th, followed by the first day of the year, brans 1st. A new day begins at sunset.
Each year is divided into four large times that correspond to seasons, each one having three months of the calendar, more or less. All months have 30 days, except bressaures which is a special division with only 5 days; it is known as gui(g) preninci "half week", because weeks are 10 days long. Each month has exactly three weeks, which means that all months start with the first day of the week, dimarn, and finish with dirols, week's last day. The days in bressaures are not named, only numbered, so dirols, 30 na cenller (the year's last big month) is followed by bressaures, which is followed by dimarn, 1 na brans.
A rough comparison with the Gregorian calendar gives the following:
1st. Brans / June 21 to July 20
2nd. Arges / July 21 to August 19
3rd. Feres / August 20 to September 18
4th. Berm / September 19 to October 18
5th. Bruine / October 19 to November 17
6th. Riçold / November 18 to December 17
7th. Mavendres / December 18 to January 16
8th. Rovanyes / January 17 to February 15
9th. Àster / February 16 to March 17
10th. Bers / March 18 to April 16
11th. Martighes / April 17 to May 16
12th. Cenller / May 17 to June 15
Half week. Bressaures / 16 to 20 June
In leap years an additional day is added at the end of bressaures, making it a half-week of 6 days. The additional day is added every four years, but every 33 years the last leap year takes place after 5 years; and so on.
To sum it up.
Megadelanean leap years in sky blue. Gregorian leap years in pink.
Megadelanean years don't exactly match with Gregorian years because those begin with the summer solstice, so the coincidence period is from June 21st to December 31st. Consider this when consulting tables like the one above.