ambiguity - Why there are two different meanings for "triweekly"?

Context: I am looking for a term to indicate a time period of 3 weeks/21 days

For instance, a "fortnightly" event would occur every 2 weeks/14 days.

My Usage:

The "Read for the Visually Challenged" is a {fill in word for 3 weeks} event as part of CSR initiatives organised by our comapny.

When I searched online, I stumbled upon Triweekly, an adjective with two entirely different meanings

1. occurring or appearing three times a week


2. occurring or appearing every three weeks

[Merriam-Webster]

Ideally, I mean #2, but will this create a confusion? Why does it mean 2 different time periods? Is there another word to indicate a period of exactly 3 weeks?

P.S: I am not looking for "monthly" since this event is conducted every 3 weeks and it could be possible that it occurs in the first and fourth week of the same month.

Answer

Confusingly (according to dictionary definitions) the same is also true of biweekly; bimonthly; and biyearly. All of them can mean once every two... or twice per...

In the case of biannual the OED gives its adjectival meaning as once every two years, but when used as a noun as meaning the same as biennial, i.e. every other year. Semi-annual can be often used to express "twice per year".

In view of the dichotomous meanings of biweekly, triweekly etc. all I can suggest is that where there is likely to be confusion that you avoid their use, in favour of once every three weeks etc. The OED recommends the use of the terms semi-weekly, semi-monthly, semi-annually to avoid any confusion when twice per... is intended.