The problem with the 'sesqui' prefix is that it is a contraction - of the Latin words 'semis' and 'que' - which means 'a half in addition'. In other words, it denotes a ratio of 1� to 1. Thus, 'sesquicentenary', 100 plus a half 100, works just fine as regards meaning 'pertaining to 150'. The problem comes when you try to apply it to any other
half-century. For example, a 250th anniversary would have to have a base unit of 166.666 and a 350th one a base of 233.333 and so on. Try saying any of these
in Latin! 750 is easier, since, by the same token, it needs the nice round base-unit of 500, giving 'sesquicinquecentenary'...ie belonging to the 500th plus half the 500th'.
Since you'd be unlikely to find any of the three words for 750th you've been offered so far in a dictionary, one has to ask, Fitzer, what you want such a word for, given that '750th anniversary' works perfectly adequately!