Calculate excel SUM of a column for days on each week

I have an Excel sheet with two columns:

• A - contain the days of the week (Mon, Twe, Wed, Thu, Fri - maybe rarely even Sat and Sun)
• B - contain a number

The records may contain not a fixed amount of records for each day, so for example `Mon` can appear in 3 consecutive rows, while `Tue` only once. So for a week we have more records than the days in one week. Also the days of the week in column `A` are sorted.

What I want is the sum of those number based when week changes. By change of the week it mean when we have on first record `Fri` and in the next `Mon` or `Thu` and `Mon`.

How can I write a formula in `C` column that it shows the result in the last day of each week? • So really the trigger is when the cell below = Mon but current cell not = Mon? – Eric F Jan 25 at 16:00
• Is the first day of data always a Monday? For example, could you ever run into a situation where one week had data for just Monday and Tuesday, and the following week's first entry was for Wed? – fixer1234 Jan 25 at 22:38

Use the following formula in `C2`:
``````=IFERROR(IF(ISBLANK(A3),SUM(B\$2:B2)-SUM(C\$1:C1),IF(MATCH(A2,{"Mon","Tue","Wed","Thu","Fri","Sat","Sun"},)>MATCH(A3,{"Mon","Tue","Wed","Thu","Fri","Sat","Sun"},),SUM(B\$2:B2)-SUM(C\$1:C1),"")),"")