Hello, paul. In situations like this, there is often a legitimate choice between spellings. Dictionaries' research can give different results where it's about 50 - 50. For instance, OxfordDictionariesOnline gives only multi-day. Wiktionary is better, saying that both variants are acceptable. // New compounds and similar forms (eg as here with a ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.