[Wavytone]

As mark suggested, spring and autumn - and around 2-4am when wind strength is usually at its minimum.

Local terrain counts for a lot too, with respect to micro-climate, and an understanding of how the wind changes direction between day and night.

Fior example, if you live near cliffs that catch the afternoon sun they will warm and produce thermals all evening that degrade the seeing above. Thermals also drift downwind and if the prevailing evening airflow blows the thermals over your site, poor seeing will affect you for most of the sky, till the rocks cool down (early morning).

Another influence is the type of terrain around your site. For example farming country or suburban areas with extensive gardens, grassed parks or sports fields mean dark soils which absorb lots of heat during daytime and then release it at night causing thermals and poor seeing, as well as moisture (as they are watered). So expect fogs and frosts if inland. Conversely light sandy soils or sandstone terrain (even better) don't build up so much heat (less thermals) and drain very freely, so fog or frost in these areas are minimal and the seeing will be better.

As for fog, even if it dies firm on a plateau, it is more dense than dry air so it tends to flow downhill. Hence better to be on a ridge than a flat location.