From the Wikipedia article on leap years, it appears that calendar systems that predated the Romans accounted for leap days, but do we know which calendar was the first to introduce a leap day, and if so, which year in human history was truly the first leap year?
Julian calendar was introduced in 45 BC. Leap years are those whose number is divisible by 4. However, there is a catch here. Of course Julius did not count the (negative) years BC (as we do).
(There is a joke: "Archeologists found a coin with the date inscribed: 45 BC":-)
To preserve the pattern of divisibility by 4, one needs to count from zero. But historians do not like zero, so in our common count year 1 is preceded by year 1 BC. (Which is more reasonable to call 0). So what is commonly known as 45 BC is really year -44. Which is divisible by 4. So this year was the first leap year.
By the common count (used by historians) it is 45 BC.