Whether you’re doing a forecast or looking back at how your sales were over a period of time, it’s important to ensure that you’re comparing apples to apples. While monthly and yearly numbers won’t have too much noise, once you’re trying to do a daily or weekly sales analysis, that’s when things can get a little challenging.

Below, I’ll show you how you can do a weekly sales analysis where you’re comparing the same days of the week against one another. This will give you an accurate picture of your year-over-year performance.

**Step one: determine which day of the week you want to start on**

This is a simple step and you’re probably going to go with either Sunday or Monday. But it’s an important one to consider because when you’re looking at weekly sales numbers, you want to be consistent. And while you can refer to the week number when comparing one week to a previous year, saying week 32 is not going to be as useful as saying the week starting August 5 or ending August 11.

In my example, I’m going to use Monday as my starting point to ensure that I’m not breaking up the weekend (the default in Excel is Sunday). To make it easy to compare a week, it will be helpful to create a header for the days of the week so it looks like a calendar.

**Step two: entering the first date of the weekday you selected**

The first Monday of 2020, wasn’t until Jan. 6 this year, which would be the second week of the year if we start on Mondays. The previous Monday was Dec. 30, which was technically week 53. Weeks 1 and 53 are often abbreviated. For now, just accept that there’s no Monday in Week 1 of 2020. I’ll show you how we can get around this problem further down.

For now, Jan. 6 will be our starting point which we’ll call Week 2. Now, that we have our starting point, we can build out what our subsequent weeks will look like.

For example, if I want to find out the start date for week 40, what I can do is simply use the following formula:

First, I multiply 7 by the difference in weeks. Then, add that to the first Monday value. In this example, it tells me the 40th Monday of the year is Sep 28, 2020. That’s why setting up the first Monday values is important to ensure that it’s easy to get the remaining dates.

This is the easier approach to take. However, later on I’ll show you a way where you don’t have to enter in the first Monday of the year.

**Step three: filling in the remaining dates of the week for your sales analysis**

Getting the starting date of the week is the toughest part. From there, all you have to add is just add 1 to each subsequent day:

Just adding 1 to the previous date will increment to the next day. No special formulas needed here.

**Step four: getting the prior-year date**

To get the previous year’s data you can follow the same approach as in step two. However, I’ll use this as an opportunity to show you another way that you can get the data. One that won’t require you to pull out the calendar.

First, we need to know what day of the week Jan. 1, 2019 fell on. To do this, we can just use the following formula:

=WEEKDAY(“Jan 1, 2019”,2)

The reason I put the number 2 as the second argument is because my week is starting on a Monday. If I set it to 1 or left it blank, the default would be Sunday. This is important because if Monday is my first day of the week then it’s day value is 1 and Sunday is 7. Had I used Sunday, then Sunday would have a value of 1 and Monday would be 2. This is why it’s important to know which day of the week you want your week to start on.

In 2019, Jan. 1 fell on a Tuesday, and the formula above gave me the result of 2. (Monday is 1, so Tuesday would be 2). The reason I need to know the weekday is because I need to adjust the date to find out when that week actually started. I use the following formula to do that:

=DATEVALUE(“Jan 1, 2019”)-(WEEKDAY(DATEVALUE(“Jan 1, 2019”),2)-1)

What this formula does is subtracts Jan 1, 2019 from the number of days it is above day 1. It then moves the date back. I can simplify this formula by entering Jan 1. 2019 in cell A1. Then my formula looks like this:

=A1-(WEEKDAY(A1,2)-1))

I no longer need to use the **DATEVALUE **function and now it’s a bit easier to use. There’s also less chance of an error when entering the date. Now, when I want to find out the first day of the week, I can multiply 7 times the week number and add to this calculation:

=(A1-(WEEKDAY(A1,2)-1))+(7*(B1-1))

B1 is the week number. In this example, if I were to enter Jan 1. 2019 for cell A1, that would give me a result of Dec 31. 2018 for the start of Week 1. Excel also considers this to be the week that contains Week 53 and Week 1. This is where you can get around this issue. By calling this Week 1 of the current year and including December’s days into this week, it will ensure you don’t have the Week 53 problem. It may not look great to call the previous year’s dates part of the new year but it avoids having to manually make adjustments for this period.

Using the updated formula, I can change the Jan. 1 date to reflect 2019 and use week 40 to update my comparables for the weekly sales analysis:

From here, it’s just a matter of now using a SUMIF function on your data to pull the sales for each one of these dates and you’ve got your comparable sales numbers. With 2020 being a leap year, you can see that the dates have moved up two days from the prior year. Without the date adjustment, you could have ended up comparing a Sunday (Oct 4, 2020) against a Friday (Oct 4, 2020).

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