Weekly calculations in both the Math Toolbox Functions and the Spread Report Functions typically allow the beginning and ending date to be defined by a number n in the calculation definition.
Table below describes supported values of n. You may also want to look at this example article that visually portrays the differences between supported values of n.

n	Beginning On	Ending On	Notes
1	Monday	Sunday
2	Tuesday	Monday
3	Wednesday	Tuesday
4	Thursday	Wednesday
5	Friday	Thursday
6	Saturday	Friday
7	Sunday	Saturday
8	1st 8th 15th 22nd	7th 14th 21st Last Day of Month
9	1st 8th 15th 22nd	7th 14th 21st 28th	Days greater than the 28th are ignored
10	1st 8th 15th 22nd 29th	7th 14th 21st 28th Last Day of Month
11	Monday (or first of the month)	Sunday
12	Tuesday (or first of the month)	Monday
13	Wednesday (or first of the month)	Tuesday
14	Thursday (or first of the month)	Wednesday
15	Friday (or first of the month)	Thursday
16	Saturday (or first of the month)	Friday
17	Sunday (or first of the month)	Saturday
18	Sunday	Saturday	Always calculates Sunday to Saturday. This is also called the "Wednesday Rule".  Whatever month contains the Wednesday is the month that will get the weekly stat. Whatever month has the Wednesday, has at least 4 days in that month, therefore, it is the month that contains the greatest number of days for that week.  The data value is placed on Saturday unless the last week of the month spans into the next month and the Wednesday is in the current month.  In which case the data value will be placed on the last day of the current month. For an example of WEEK18 see article 10539
19	Sunday	Saturday	Always calculates Sunday to Saturday. Data is considered to be part of the month that the most data falls into. If there is equal data from each month in a single week, the data is placed on the Saturday
20	Sunday	Saturday	Calculates only complete weeks within the month.
22	1st 8th 15th 22nd 23rd,24th, or 25th	7th 14th 21st 28th Last Day of Month	The fifth week is the last 7 days of the month.  If the month only has 28 days, the fifth weekly average will not be calculated.  Used in Ohio.
23	Sunday (or first of the month)	Saturday (or end of the month)	Sunday thru Saturday, the last week of the month is Sunday thru the last day of the month.  The first week of the month is the 1st thru Saturday.
24	Sunday	Saturday	Sunday thru Saturday, values are calculated on the Sunday (i.e. the start date of the week).
99	Start Date of Report		The start date of week 1 is the start date of the report, week 2 starts 7 days later... ONLY SUPPORTED IN SPREAD REPORTS.