Day of Rest

The best places are those where you feel you really need a rest day!
From Burgos to Leon is not very taxing although the mesta only really starts once you've climbed an impossible little hill after Castrojeriz. You could spend a rest day in Leon - that is about 1/3 of your trip to Santiago.
After Leon you start climbing to the highest point on the whole Camino which is just after Manjarin - 1517m. Then a very steep downhill to el Acebo.
You could spend a rest day in Villafranca del Bierzo - before you climb another steep hill to O Cebreiro - or at O Cebreiro itself which is close to the next 1/3 mark of your ride.
Play it by ear and see how you go. You might want to do a short day and rest up for the remainder of the day rather than take a whole day off.

As ever, I absolutely defer to the wisdom and insight of Sillydoll.
Entirely right - rest when needed.
I would enter a plea for the Maragateria - the lovely area before amidst the hills after Astorga, great places to recoup energy.
Cacabelos in the Bierzo is wonderful - specifically the local produce which can be sampled at Prada a Tope, for example.
http://www.pradaatope.es/tienda/

Yes. Sillydoll is the expert. Leon is a lovely laid back city if that's the kind of thing you want.

However, you might just want to be adventurous and get off route for a day or so to explore the depths of Spain! As a yellow arrow junkie I can't really advise on that.

Buen Camino!