A secular year is the year that closes a century. The secular years are multiples of 100, and therefore end up in two zeros: 100, 200, 1500, 2000, etc.
To avoid time gaps, the Gregorian calendar states that only secular years that are divisible by 400 will be leap years. For example, the secular years 1700 and 1900 were not leap years because they are not divisible by 400, but the 1600s and 2000s were.
According to the Gregorian calendar, which came into force in 1582, the leap year secular years were and will be: 1600, 2000, 2400, etc.
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