When does the first week of a given month start?

Question

I wish to display a bar chart showing weekly revenues per month. A week is defined from 12:00 AM Sunday to 11:59 PM Saturday. I want to display full week values (i.e. not just revenues for 7/1 for first week of July). When should the first week of a given month start and the last week end, according to practices that have gained widespread acceptance? Here are several options:

First week starts when any day of a given month falls in that week:

• July Week 1 from 6/25 to 7/1
• July Week 2 from 7/2 to 7/8
• July Week 3 from 7/9 to 7/15
• July Week 4 from 7/16 to 7/22
• July Week 5 from 7/23 to 7/29
• July 30th and 31st are part of August and not part of this month.

First week starts when all days of the given month fall in that week:

• July 1st is part of June and not part of this month.
• July Week 1 from 7/2 to 7/8
• July Week 2 from 7/9 to 7/15
• July Week 3 from 7/16 to 7/22
• July Week 4 from 7/23 to 7/29
• July Week 5 from 7/30 to 8/5

Answer 1

Less Ambiguous:

The first full week of July

That would be less ambiguous, though you'd still potentially confuse people if the 1st of the month is on a Monday... Cultures differ in if Sunday or Monday is the start of the week.

Unambiguous:

The first full week of July (2nd—9th).

This explicitly provides the dates mentioned in the range. Also, this helps the user by providing the dates explicitly so they don't need to reference a calendar if they know the date they have in mind. For example,

I know this event happened June 8th... Was that the first or second week of June?

Answer 2

Use the same definition for weeks of the month as for weeks of the year.

In the US, which is the locale to be assumed by the contextual data provided, this would mean that the first week is the one which includes the first day of the month or year. There are, however, some who prefer to use partial weeks (down to a single day long), while many others prefer full weeks, but those then split into a group of people who limit a week to be assigned to a single month and year only and a group that is okay with double assignments. Anyway, full and unambiguous weeks are required by this scenario.

Internationally, the well accepted standard ISO 8601-1 and its predecessors have long established the convention that there are only full calendar weeks (i.e. either 52 or 53 in each year). They belong to the year that most of its days belong to, i.e. 4 to 7 and this principle is easily extended to months or quarters. Since Monday is the first day of the week here, this is always equivalent to the larger entity its Thursday is in. Likewise, the 4th day of the month or year is part of its first week. Last weeks follow from that.

Unicode CLDR data includes information which convention is preferred where, although it is somewhat changing over time and not yet empirically correct for all places on Earth. It also collects linguistic data for the possible and preferred designation of weeks:

• ordinal week 26
• the first week of July
• the first week in July
• the week of 1 July
• the week starting 25 June
• the week from 25 June through 1 July
• and so on