The word you're looking for is "quasquicentennial," although it may
not have found its way into every dictionary yet, according to this
"In addition to "centennial" and "millennial", normally used for the
100th and 1000th anniversaries of events, one may add the series of
Latin prefixes for 2 through 9 above to either word to indicate 200th
through 900th (or, more rarely, 2000th through 9000th) anniversaries.
But some people can't wait 100 years to celebrate. In North America in
the late 19th century, when many cities, churches, etc., were over 100
years old but not yet 200, the term sesquicentennial (150th
anniversary) was coined (from Latin sesqui-, of the ratio of 3 to 2).
Even fifty years was too long for Delavan County, Illinois, whose
leaders consulted Funk and Wagnall's in 1962 for a word for a 125th
anniversary, and were told that quasquicentennial, an irregular
formation from Latin quadrans, one-quarter, might do, but that it
wouldn't get into the dictionary until it became more regular. It's
now in the Oxford English Dictionary, and has been used quite a few
times since, mainly in the American Midwest."
Here's a confirmation at "information please":
Also, if you're looking towards the future, you might be interested in
these Latin anniversary terms as well:
Thanks so much for the interesting question today. May we all live to
enjoy our own quasquicentennial one day, medical science and God