That's when we do it now. Not a date I picked; it just turned out to be the only day everyone was already home, already fed, and not going anywhere.
Newspaper down over the table. Music on. Everyone still full from the day before.
The gel goes into the mold. Water on top. And the shape forms right there, in front of them, in about a minute — no oven, no overnight, no "let's see how it looks tomorrow." My youngest was six the first year and did every step herself, start to finish.
That last part matters more than it sounds like it should. There are no tears on a Sunday in November, because nothing collapses, nothing cracks, and nothing has to be started over.