I was just reading about Saudi troop deployments for this year’s Hajj. Something like 2 million people from 140 countries will follow in in the footsteps of Muhamed this year. It is the largest annual migration of people on the planet. That got me thinking about other large movements of people throughout the world. What other mass migrations take place in the world today? I don’t have any particular criteria in mind, but let’s say, more than 10,000 people travelling more than 100 miles to and from their homes on a somewhat regular basis. That would rule out things like one-time movements of refugees, or suburban commuters.
Any major holiday at any major university.
Berkeley alone has over 33,000 students, more than half of which are from southern California, and the university is, for all practical purposes, empty during the Thanksgiving and Winter breaks, not to mention every August and May. One of these dates is a national holiday, and the other is (largely) a cultural/religious holiday. I suppose you could throw in Spring Break, as well, when hundreds of thousands of students make pilgrimages to vacation spots. Or maybe Mardi Gras. I suspect that it is partly this commuting, in addition to Stanford and Davis, that keeps Southwest Airlines alive. 
I don’t suppose that counts, but it is a cultural ritual for American youth (and their parents).
On a more serious note, you also still have several nomadic/migratory cultures in central Asia (whose numbers are rapidly dwindling). There are still seasonal workers who travel from Mexico to work in the US at busy times, especially in agriculture, and go back.
- 33,000 undergrad students, buts some thousands of grad students.
For perspective of how empty the city becomes, the total population hovers around 100,000.
The Hajj pales into comparison with India’s Kumbh Mela religious festival. 130 million people are estimated to have attended the festival in 2001. I suspect that figure is an overestimate but a bit of searching should provide you with more reliable figures. It’s not an annua; event but occurs every twelve years (from memory)
See story here
That should be annual