Will skies be clear over southern Ontario Saturday night?
Partly clear skies is the best description for skies that will most likely have some cloudiness at low and mid levels. The illustration below shows some moisture forecast at mid levels during the evening, but it should be considered as an example of the situation at most levels, any time of the night, since some moisture will be present to cause some clouds in the sky over any particular region.
So, in summary, partly clear skies are likely with some cloudiness.

Will skies be clear over southern Ontario Thursday night?

Mostly cloudy skies seem likely, except for less cloudiness in regions near and north of GTA, and eastward over eastern Ontario. A region of moisture at lower and mid-levels (illustrated below) is forecast to move eastward over southern Ontario Thursday night ahead of a weak mid-level trough and this moisture is likely to appear as cloudiness. Regions near and east of GTA will likely have less cloudiness during the evening, until the moisture and cloudiness moves eastward over eastern parts of southern Ontario later in the evening.
In summary: partly clear skies near and east of GTA during early evening, mostly cloudy elsewhere over most of southern Ontario.

Will skies be clear over southern Ontario Wednesday night?

It looks like skies will most likely be clear over southern parts of southern Ontario during the first several hours after sunset, and then become mostly cloudy by about mid-evening. Elsewhere, skies are likely to be cloudy by evening. 

Clear skies following Wednesday afternoon’s cold front will be followed by increasing low and mid-level moisture which is forecast to move eastward across southern Ontario during early evening (illustrated below at EST 19:00) and the moisture is likely to cause skies to become mostly cloudy over most or all regions by about mid-evening.

Will skies be clear over southern Ontario Monday night?

Some clear skies are likely over some regions during the early evening, due to the dry air causing the partly clear skies Monday afternoon, until more extensive cloudiness spreads over most regions by late evening. Some mid and high-level clouds will most likely be visible during the early evening, due to moisture at mid and upper levels over many regions (illustrated below for the 500 and 700 hPa levels) as well as some patches of uplift (not illustrated) contributing to form clouds from the moisture. Low-level moisture is likely to be more widespread later in the evening (illustrated below for late evening at the 925 hPa level) and this is the main factor that is likely to cause more widespread low-level clouds later in the evening over most regions of southern Ontario.

Will skies be clear over southern Ontario Friday night?

It looks like skies will most likely be clear during the early evening over most regions, followed by increasing cloudiness by midnight.

During early evening, a weak high pressure ridge over southern Ontario will be contributing dry air and diminishing winds, until high-level moisture moves eastward over southern Ontario (illustrated below for early evening) ahead of Saturday’s low pressure trough. Mostly clear skies are likely, except for regions southeast of Lake Huron and Georgian Bay, where lake-effect cloudiness may cause cloudy skies during early evening.

More substantial mid-level moisture that is forecast to move eastward (illustrated below for the hour after midnight) is likely to cause cloudiness over southwestern regions by late evening and all regions by or after midnight.

Will skies be clear over southern Ontario Monday night?

Skies could become partly clear for several hours after sunset over some regions, before widespread cloudiness moves over southern Ontario later in the evening. However, the break in cloudiness after sunset will most likely have some mid-to-high level cloudiness present (illustrated below for high-level moisture at the 500 hPa level) but skies may be clear enough for brighter stars and the waning gibbous moon to be visible and potentially give the impression of clear skies.

Will skies be clear over southern Ontario Saturday night?
Clear skies are likely to continue over most regions until high-level cloudiness starts to move eastward over southern Ontario later in the evening.
The images below illustrate the high moisture that is forecast to move over southern Ontario ahead of a low pressure trough which is expected to move over the central Great Lakes region by Sunday morning. Meanwhile, south of Georgian Bay and Lake Huron, more extensive lake-effect cloudiness is likely to cause low-level cloudiness during most of the night.