Conversely, what about: {date} is in the last week of {month}?
Is the first week of April the first week that is entirely in April or is it the first week that contains the first of April? Of course this all changes when considering whether the start of the week is Sunday or Monday.
There may not even be an "answer". In that case I'm interested in the most common usage.
Edit:
This SO post and this mailing list are what have caused me to call the meaning into question in the first place.
