when can you perform skywave propagation?

Yes, it doesn't depend on the time of the day.
Radio waves travel in straight lines so they can't directly be heard over the horizon (~40 miles but it depends on the antenna height above ground). However, there are several layers in the upper atmosphere that are to some degree reflective and waves hitting them will be bounced back, often making the signal reach beyond the horizon although not necessarily along the whole distance. There can be gaps before the reflection lands again. How reflective the layers are depends on many factors including temperature, air pressure and radiation levels from the Sun. The layers tend to contract (become closer to the ground) at night as it cools down and then expand back again during the day. If you listen on a MW domestic radio you will notice it picks up far more stations at night as the reflective layers get closer,

Brian.