Is it possible to set corner stones / L-shaped stones in winter?

We mixed the relatively lean concrete for our L-blocks (mostly 2m high), meaning little water. If you, for example, lay the foundation in the morning and set the blocks in the afternoon, frost down to about -3 degrees at night is, in my view, not a problem. The curing process itself also generates some heat. Furthermore, the foundation is sheltered from the wind at night thanks to the L-block and the soil. How many meters do you want to set? If you're under time pressure, I would take the "risk"; if not, wait.

There are also antifreeze agents for concrete.

Down to -5°C it should be unproblematic.