An interval of 300 years.
It is called a 'quincentennial' Quin= five. Centennial= a period of 100 years.
Penry antennary venery bicentenary catenary centenary octocentenary plenary quatercentenary quincentenary quinquecentenary sexcentenary tercentenary tricentenary near rhyme (spoken) consider "Em-ree" endings e.g. memory emery ephemerae