Except this doesn't retain the crucial information: why? It looks arbitrary. The thought that "some countries have sales tax rules dependent on the day of the week" may or may not be obvious from the context. At the very least, the comment pins a point in the space of all possible reasons for that piece of code - with it, you know it's related to sales tax and week days, and isn't e.g. a workaround for the bug with NaNs in tax rates that you saw on the issue tracker last week.