Dictionary.com says both. See link. *shrugs* As I said to @Carlos Hazday, it's probably also a regional thing as well as both @Valkyrieand I are near each other in the "Inland Northern" dialect region. https://www.washingtonpost.com/blogs/govbeat/wp/2013/12/02/what-dialect-to-do-you-speak-a-map-of-american-english/
For those that didn't explore the more information link... probably depending on where you are, they use:
Semi-annual for 2x a year
Biennial - for every two years.
Now that the forums are consolidated, one of the features added to the system years ago is a LOT more useful. You can now select Fluid View and see all topics in a single list, instead of having to go to different forums to see the most recent topics. You can see them all in one go and filter to see only the forums you are interested in.
If you are not a fan, you can still use the Traditional view, which is the button next to the one with the giant red arrow, shown above.