It's hard to say exactly what the weather will be like in the Scottish Highlands at any given moment for two reasons. First, it is a pretty large area, and characterizing one section of it may not be true for another. The second reason is that the Highlands find themselves in an interesting position in Europe. The Scottish Highlands are located between the Continental land-mass and the Atlantic maritime conditions. Because of the combination of high and low pressure weather systems between the two, the weather is constantly changing and unpredictable. There isn't necessarily a "good" time of year to visit because it depends on what you are interested in. You can visit in the spring and summer time, and you will get some warmer days as well as more sunshine. You can do hiking and camping outdoors. However, you could also visit in the fall and winter and be able to go skating and skiing. Either way, you shouldn't expect outrageously warm springs and summers, and you should pack accordingly. Definitely bring pants and a jacket when you visit.