What Day of the Week Will It Be 100 Days From Now?

Now that it’s heartening to think about what we might be doing 100 days from now, let’s use some basic math to figure out what day it will be 100 days from now. Here at Adafruit we use math all the time for all sorts of things. Math isn’t just useful, it’s a language we use to talk to each other. And that’s not just true for makers and DIYers — it’s true for all of us! Here’s a wonderful use of modular arithmatic to determine the day of the week in 100 days from Finding Gemstones.

As a better approach, we know that every 7 days, we will be back to the same day of the week. So if you start on a Thursday, after 7 days it will also be Thursday, and after 14 days it will still be Thursday, and so on. The smallest multiple of 7 less than 100 is 98, so 98 days from today, it will be a Thursday. (Yes, I am writing this post on a Thursday.) Therefore, 100 days from today it will be a Saturday.

What we’re really doing here is computing the remainder when 100 is divided by 7. The quotient is 14, and 7 times 14 is 98, so the remainder is 2. So the day of the week it will be 100 days from today is the same as the day of the week it will be 2 days from today, namely Saturday. This is the basic concept of modular arithmetic, the art of computing remainders.

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