Random Riddle: A Time To Sow

I was created in 46 B.C.
Brought into being by Caesar’s decree

But still the seasons were unimpressed
Man-made and tropical slowly digressed

So good Pope Greg set everything straight
with inspired rules that worked out great

I am grander than my common peers
I come 97 times every 400 years

By following my rules, as widely agreed
We all know what day to plant our seed

What am I?