JULIAN CALENDAR
It was in 46 B.C. that Julius Caesar ordered adoption of the Egyptian Solar Calendar of 365 days and taking cognizance of the additional fraction.
Assuming that this fraction is 0.25 day, the Julian Calendar provided for the regular intercalation of one day after every 4th year.
But, there was a source of error in the estimate of the odd fraction as 0.25 day rather than .2422 day per year. By the year 16th Century the accumulated error had amounted 10 days.
In 1582, Pope Gregory XIII ordered the so-called Georgian reform omitting 10 days of that year (the day following October 4 was declared October 15) to put the calendar and sun into correspondence again.
The Georgian reform also prescribed that in future the additional day may be omitted from all years divisible by 100 expect those by 400. Thus 1700, 1800 and 1900 were 365-day years while 1600 was a leap year and 2000 will also be a leap year.
This adjustment also is not free from error. It covers intercalary or accumulation of days up to 3rd decimal while the error in the 4th decimal will amount to a full day in about 3,300 years.
In short the Solar Calendar needs adjustment after every four years and still it will accumulate one-day difference in 3,300 years. On the other hand the Islamic calendar will require adjustment after 2541 years when it will gain one day because of the omission of 34 seconds in calculation every year.