triennial

1. The 300th anniversary (or the celebration of it) (synset 115276568)
   is a type of: anniversary, day of remembrance - the date on which an event occurred in some previous year (or the celebration of it)
   same as: tercentenary, tercentennial

1. Occurring every third year or lasting 3 years (synset 301976671)
   similar to: periodic, periodical - happening or recurring at regular intervals

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