tercentenary

A 300th anniversary or its celebration.

Of or relating to a span of 300 years or to a 300th anniversary.

Comprising three hundred years; including or relating to the interval of three hundred years.

A day observed as a festival in commemoration of some event, as the birth of a great man, or a decisive victory, that happened three hundred years before: as, the Shakspere tercentenary.

Including, or relating to, an interval of three hundred years.

the 300th anniversary of an event

​of, or relative to such an anniversary, or to a span of 300 years

the 300th anniversary (or the celebration of it)