When Is the Best Time
Isle of Skye, also known as "Misty Isle", has only a few really good days for climbing the higher mountains. The best time is usually from mid-April to the end of May, but you can be lucky and experience a great day throughout the whole year. From my experience, it is better to start afternoon, since evenings are usually more calm on Skye. Check the forecast before you go! Note that climbing Sgurr Alasdair requires mountain experience. People who suffer from vertigo should not try this.
Note that Cuillin ridge can be a very dangerous place for people who underestimate it. Weather can change very quickly and navigation is very hard on the ridge. Use proper clothes, shoes and equipment when going to the ridge and be ready for every eventuality. Midges are very annoying from June to September and yes there are plenty around Glenbrittle. It should be better in the mountains though.
For detailed weather and crowds information check out my article from Stac Pollaidh one of the Assynt Hills.
Best Months to Visit
Location and Tips
PLEASE NOTE that climbing this mountain can be very dangerous. It is not a walk and demands scrambling. Check the forecast and don't go in wet conditions or bad weather unless you are an experienced climber. A lot of people get lost or injured on the ridge every year. I recommend gathering some info about the place or go with someone who knows it.
Sgurr Alasdair is 992 meters high and the highest summit of the whole Cuillin ridge. The easiest way up starts in Glenbrittle campsite. Go around the toilet block through the gate and then up the hill, don't go to the right. After a while, there is a junction. Take the left path. Soon, you can see Sgurr Dearg on the left side, Sgurr Mhic Choinnich in the middle and Sgurr Alasdair on the right side (the sharp one). Continue walking the path and soon, you will get under Coire Lagan. Climb your way up to the loch (little scramble). From the loch, climb the Great stone chute. It is a very steep part full of stones and scree (some parts are unstable). Be very careful, especially in the upper part and when there are other climbers below you! After some time you will get to the col and the views start to be stunning with southern Cuillin. Go right and scramble your way to the top. On a clear day, you can see the whole ridge from here. It is simply one of the greatest places to be. Descent the same way you went up. More info here: http://www.walkhighlands.co.uk/skye/sgurralasdair.shtml Short video from the summit.