STUCCO NEWS.NET

The rule is that mortar should be kept above freezing until it is hard enough to knock on.
My rule comes from old bricklayers and plasterers. I know this is hearsay, but I believe people who work in the trades. This is how I learned. Usually 5 to 7 hours is plenty of time with no accelerators to set properly before the temperature drops below freezing. The freezing temperature for mortar that is setting up is about 28 degrees. Bear in mind cement and concrete generate some heat while they are setting up.
If cement mortar is allowed to freeze before it sets, it turns to powdery mush. One can write his name with his finger in frozen mortar after it thaws. Bricks and blocks can be lifted back out of the wall.
I use heaters and tents in cold weather, but there are times when one pushes the material to the limit. Generally, if we put stucco on an occupied building with the heat on, the wall temperature is above freezing just from heat escaping from the building. Also, cement mortar and does generate some heat while it is setting up.
In warm climates none of this is a concern, but here in Northern Virginia and the Washington, DC area, cold weather protection is a necessity.

Using Gypsum plaster as an accelerator in cement stucco:

I use Gypsum plaster as an accelerator but very sparingly. Plaster is Calcium Sulfate, the ingredient in Portland cement that forms crystals and sets up everything else. Molding plaster is used in pre-cast concrete blocks, known as stones. Molding plaster as an accelerator allows several stones to be cast in a day, instead of one a day. Gypsum is said to weaken the concrete. but probably not much, if at all.
Plaster should never be used in a stucco finish coat, ever. It will cause efflorescence.
My preference is to use brown mortar, or basecoat plaster with no sand. It sets slower than gauging plaster or Molding plaster. Gauging plaster and Molding plaster are both white plaster of Paris, the only difference is that Molding plaster is ground finer.
Cement mortar using plaster as an accelerator should never be re-tempered, and should be thrown away when it sets. My rule is to use less accelerator than you need, instead of too much. It is better to stand around and wait for mortar to set up than to mess up your whole wall.
Plaster should not be used as an accelerator for cement mortar in government work unless it is on the approved material list.

A word about heaters for stucco:

Stucco in the snow.

A blower type heater works well, directing the heat allowing work to proceed without a tent.
Some propane heaters come with a fan to circulate the heat.
Propane costs less than kerosene,
but it does require a lot of time and
labor to take those heavy tanks to be filled.
One should never use diesel fuel.
It stinks so bad and it will gag you out.

More about accelerators:

Portland type III, (high early), sets fast, and if it available, is a good alternative to accelerators. We use it to build up thick areas like window surrounds in cold weather.

Calcium chloride is good as an accelerator if used no more than 2%. It was used a lot in
plastering swimming pools, where two coats and same day completion is critical for a good bond between coats and uniformity in appearance. The new swimming pool plasters premixed with acrylics have made calcium chloride obsolete, except for maybe building up round shapes and thick areas.
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I hope this is of use to somebody and  hasn't arrived to late for this year. Next winter will be here right on schedule.