Show HN: Weekle – a web app to learn how to calculate the day of the week

Dividing by 7 is also a cool trick and it’s way easier than this.<p>1&#x2F;7th is 0.142857 recurring. All you need to remember is one-four-two-eight-five-seven. Say it over and over. It’s my favourite number.<p>The magic: to get 2&#x2F;7, 3&#x2F;7, <i>any</i>&#x2F;7, just move the decimal point:<p>- 2&#x2F;7: 0.285714285714…<p>- 3&#x2F;7: 0.428571428571…<p>- 4&#x2F;7: 0.571428571428… (I’m typing these out but ‘from memory’ but not really: only by knowing 1-4-2-8-5-7)<p>- 5&#x2F;7 &amp; 6&#x2F;7: left as an exercise for the reader.<p>So then the trick is to play a bit of mental magic. Pretend like this is really hard and that every decimal point is stretching your mental capacity to its very edge.<p>31&#x2F;7? That’s 4, with 3&#x2F;7 left over. Say it out loud as you gaze in to the middle distance and do this difficult, difficult calculation in your head: “four point four two eight five seven … one four … two eight five … seven one four …” and so on.<p>A fun trick which is very occasionally practically useful.

First question was “08&#x2F;10&#x2F;2023”<p>The majority of the world uses little endian, so the 8th day of the 10th month of the 2023rd year. 8th October is Sunday, as I know that because it’s the same day as 9th of May.<p>However experiments E has told me that much of the internet uses the minority American view of “middle endian”<p>Probably worth making it clear.

I&#x27;ve found the Month&#x27;s First Sunday method[1] (a mild simplification of the Doomsday Algorithm including the odd-plus-eleven innovation) easiest, fastest, and most practical. I&#x27;m not insisting on it, just great for me, and I wish it were better known.<p>[1]: <a href="https:&#x2F;&#x2F;firstsundaydoomsday.blogspot.com&#x2F;2011&#x2F;01&#x2F;learn-by-example.html?m=1" rel="nofollow">https:&#x2F;&#x2F;firstsundaydoomsday.blogspot.com&#x2F;2011&#x2F;01&#x2F;learn-by-ex...</a>

My seven year old thinks I have superpowers now. Needless to say, I’m a big fan - it’s pretty amazing to go from learning to applying as quickly as I did. Great job!

Love it, why can I not choose YYYY&#x2F;MM&#x2F;DD as my date format though (tbh this should be default with manual selection to anything else!)

This looks easier to memorise than another method: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Doomsday_rule" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Doomsday_rule</a><p>Nice work on the site!

Can you add more detail on the year table at the very bottom &#x2F; how you&#x27;re meant to use it? I can memorise it but I&#x27;m not sure how you&#x27;re meant to use it after that.

What do you get for Sep 3 1752?

I love the bit that sticks out on desktop with the QR code. Stellar work, Ben!

&gt; Mentally calculating the day-of-the-week for any date in history sounds like an impossible task for a normal person ...<p><pre><code> $ cal &lt;month&gt; &lt;year&gt; </code></pre> And look :-)

This was done ages ago.