Microsoft runs intern programs all the time, but there's a particularly large wave that comes for summer college internships. This is the wave that has tons of weekly events, product fairs, and a summer-end party at Bill's house. After watching this year's candidates trickle through for their interviews in the spring, the last few weeks have had a noticeable rush of those that accepted offers. I don't know how many interns there are in total, although I've read that it's somewhere around a thousand in the area. Nevertheless, the number is quite small compared to the size of the regular employee workforce. For some reason though, it is possible to spot the intern if you have a careful eye. This is because

1. They are frequently in the vicinity of the intern staples beer and pizza, a fact which attracts many non-intern visitors to their events
2. Although Microsoft employees receive enough free shirts to rarely need laundry services, only the interns are brave enough to wear them
3. The urge to go to the grocery store and get an entire cart of nothing but microwaveable dinners and koolaid packets fades quickly after graduation