When creating a report that uses 'is in calendar range' criteria, it isn't immediately obvious how the 'different ranges' for 'week' work. For example, is a calendar range of 'last week' the 7 days prior to today's date or including today's date? (or something else altogether?) etc.

All calendar range criteria that make use of weekly criteria function with Monday as the start of the week (starts at 00:00 on the Monday and ends at 23:59:59 on the Sunday).  For example, if you run a report on Wednesday 10^th with criteria of 'is in calendar range - last week', the results returned will be from Mon 1^st to Sunday 7^th, inclusive. 

As a minor note: Be aware of timezones.  Even if all your users are based in one timezone (as is the case in the UK), you may need to consider British Summer Time (BST) and Greenwich Mean Time (GMT).  This is due to the reported time when clocks go forward (in Spring) and backwards (in Autumn) by one hour.  This is just a mention / reminder, and unlikely to require consideration unless the data you're reporting on falls at a time between midnight and 02:00.  This is due to the 'hour movement' coming into effect at 01:00, which means 01:00 - 01:59 'happens twice' when the clocks go backwards:

00:00 - 00:00 BST

01:00 - 01:59 BST

01:00 - 01:59 GMT

02:00 - 02:59 GMT