frost and new brickwork

If the mortar froze, then it will never be as strong as if it hadn't frozen (I was taught that it sets no further from that point on, but I haven't ever seen any supporting evidence for this). However, a minor frost does not mean the temperature dropped below zero. There was a white frost all around me this morning, but some part-built brickwork I have which had pooled water in it from yesterday's rain had no sign of ice in the water, because the temperature actually didn't drop below

2C.

You should protect new brickwork from freezing, such as covering it up and/or using a low temperature additive in the mortar. When I was building a gatepost a few years ago and was expecting a frost one night, I put hot water into a number of 2litre coke bottles and put them in the hollow middle of the gatepost, with the gate post cap resting on top to keep the heat in (or at least force most of it to escape through the brickwork, keeping it from freezing). That seemed to work well.

Mortar takes around 6 weeks to reach full strength, and you should probably protect it for as much of this as you can for max strength. (OTOH, you rarely actually need max strength.)