The verb transpire traditionally means (1) to give off vapor, or (2) to become known, but today it is most often used as a formal synonym of happen. Some very careful English users resist this change, but it is well established and probably can’t be stopped. Strictly speaking, though, there is no use for this sense of transpire even in formal contexts, as the ancient and reliable happen always works in its place.
For example, these writers use transpire as a formal word for happen:
There are not, though, and that’s why what transpired at the Air Canada Centre really doesn’t come as the biggest of surprises. [Toronto Star]
What ultimately will transpire in Libya probably lies somewhere in between the extreme scenarios. [Forbes]
Political parties and media divide the public with rhetoric designed to distract us from what is really transpiring. [Santa Maria Times]