Finally, today I will be working on my oven floor! Is it necessary or prudent to cover the finished floor from the weather until I work again in a couple days (moving a daughter)?
Congratulations, Bill! This is one of the fun parts of the job, and very satisfying to see the herringbone pattern emerge.
Yes, this is when you need to start keeping water out of the sand underneath your firebrick. Moisture will eventually be wicked away by the concrete frame (which—defying all appearances—is porous and will suck up water), but that takes time and will affect the performance of your hearth until it happens.
Take a couple of left-over 2x6s from your base slab form, lay them on the hearth along the long sides, and lay a tarp over it. The 2x6s will keep the tarp off your new hearth floor and promote air circulation.