When Pope Gregory added 13 days to Julian Calendar !

The Tropical   Zodiac, Sayana Jyotis Chakra, moves backward one degree every 72 years. This astro phenomenon is known as the Precession of the Equinoxes.

Seasons were changing and Western astronomers went to Pope Gregory and told him that the existing Julian Calendar was erroneous and needs to be reformed. 

This old pagan Calendar had only 10 months. The Seventh Month was Sapta or September,the eighth, Ashta or October, the Ninth November and Tenth, Dasa or December.Named after Julius Caesar, this Calendar was defective and had to be rectfied.

( January after Janus, February after Goddess Februa,March after Mars, April after Goddess Aprilis, May after Maia or Mercury, June after JUno. August after Augustus Caesar et al)

The error was due to the fact that they took only decimals 365.25 for an year,
whereas the actual value was 365.256362424815, a value given by the British 
astronomer, Sir Simon Newcombe.

So Gregory empowered C Clavius to head the Calendrical Reforms Committee. With the help of Aryabhata's and Madhava's Tables of RSine Differences, they triumphed, solved their problem of 3 Ls and reformed the Calendar.  

They used the value 365.25636 = 1 year to solve the problem.