Step 3: In Cell F1 enter Average, and then enter the formula =IF(AND(D2=D1,E2=E1),"",AVERAGEIFS($C$2:$C$39,$D$2:$D$39,D2,$E$2:$E$39,E2)) into Cell F2, and then drag the Fill Handle to the range we need.

Notes:
(1) If two purchases occurred in the same week of same year, the formula of =IF(AND(D2=D1,E2=E1),"",AVERAGEIFS($C$2:$C$39,$D$2:$D$39,D2,$E$2:$E$39,E2)) will return blank; if not, it will return the weekly average.
(2) And in this formula, D1 and D2 are cells with week numbers, E1 and E2 are cells with years, $C$2:$C$39 is the range you will calculate weekly average, $D$2:$D$39 is the WeekNUM column you will evaluate by week numbers, $E$2:$E$39 is the Year column you will evaluate by years, and you can change them based on your needs.

(3) If you want to calculate the weekly averages but ignoring years, you can enter the formula =AVERAGEIF($D$2:$D$39,D2,$C$2:$C$39) into Cell F2 and drag the Fill Handle to the range you need. And you will see the weekly averages as below screen shot shown:

 This method will introduce Kutools for Excel's To Actual utility and Advanced Combine Rows utility to batch calculate all weekly averages at ease.

1. Besides original purchase table, enter Week in Cell D1, and then enter the formula =WEEKNUM(A2,2) (Note: A2 is the date cell in Date/Time column) into Cell D2, and then drag the Fill Handle to the range we need.

2. Select the purchase table with new Week column, and click the Kutools > Content > Advanced Combine Rows.

3. In the opening Combine Rows Based on Column dialog box, you need to (see screenshot below):
(1) Click the Fruit column in the Column list, and click the Combine > Comma;
(2) Click the Amount column, and click the Calculate > Average;
(3) Click the Week column, and click the Primary Key;
(4) Click the Ok button.

So far all weekly averages are calculated with corresponding fruits are listing besides the averages as below screenshot shown:

Average by year/month/date in Excel

Average timestamps of day in Excel

Average per day/month/quarter/hour with pivot table in Excel