Learn English – is there a word to represent once every nine months

Bellwether seems like an excellent usage for the situation you describe, because when you're talking about trends (at least, other than retrospectively) you're somewhat inherently talking about prediction. If prediction weren't relevant to the situation I might use exemplar or typifier, or perhaps something like quintessential example if I were wont to wax verbose.

Those are all for a single item; for a subset of items, representative sample comes thundering out of the land of statistics to dominate the lexical landscape.

Avoid biweekly altogether. Use fortnightly for "once every two weeks", and twice a week for, well, "twice a week".

Not everything has to be a single word, so don't be afraid to use more than one word when you want to use clear, understandable, unambiguous language.

If they have problems with fortnightly, use every other week, or let them into a little secret: fortnight comes from "fourteen nights", or two weeks.

Edit: From the comments it's clear many Americans won't understand or are uncomfortable with fortnight so, to be safe, use the aforementioned every other week.