The source code is very straightforward too. Commented out sections of the website with dated reminders to uncomment them. I guess he must look at his source code frequently enough that these reminders work?
Very simple but effective mechanical turk I guess?
e.g.
<!-- Enable just after Thanksgiving thru Jan 6 -->
<!--
<center><h3>
It's the Grav-Mass season!<br> <a href="/grav-mass.html">Grav-Mass</a>
is the celebration of physical laws. <br>December 25<br>
Isaac Newton's birthday.<br> ¡Feliz Gravidad!
</h3>
<a href="./grav-mass.png"><img src="./grav-mass-icon.png" alt="A Grav-Mass tree"></a>
</center>
-->