How do I calculate the number of concurrent users to use in a load test?

We’ve come across this question fairly often at Load Impact, so I thought I’d add it to the Stack Overflow community to make it easier to find.

How do I calculate the number of concurrent users (VUs) that I need to simulate during a load test, in order to stress my system with the same kind of traffic that it will normally see in the course of a month, week or day?