Existence of multi in US English Yes, the prefix multi is valid in American English, and usually used unhyphenated You can see dozens of examples on Wiktionary or Merriam-Webster If your grammar and spelling checker fails to accept it, it should be overridden manually
Multiday or Multi-day - English Language Usage Stack Exchange What is correct, Multi-day or Multiday? I have checked the collins dictionary and it does not have an entry for Multi-day but does for Multiday (link) Despite that, almost every browser or device