Given a 14-day incubation period (most lengthy estimate), if the MERS-CoV is going to show up any differently among hajis this year than it did as a result of last year's Hajj (you may have noticed that no pandemic ensued), we should see those cases having presented with symptoms by Sunday the 27th. If we really want to get a feel for how easily transmissible this virus is, then this is the opportunity to observe and test.
Of course, this is an artificial date, given pilgrims have been assembling and mingling for some time during the lead-up to Sunday evening, but I'm using that date as the outer limit for appearance of symptomatic cases using a timepoint in which many people definitely congregated together in large numbers.
We may also see a rise in cases appearing outside the KSA as returning pilgrims picking up infection just before they depart, become symptomatic on home soil. Or we may not.Speaking as a hyperchondriac (a perfectly healthy ignoramus who fears for the health of others, especially from rare diseases), I suspect the Hajj will result in little beyond the usual run of flu and colds among returning pilgrims. To risk MERS they will also have to start swarming into Saudi hospitals' ICUs, or somehow expose themselves to the still-unknown outside source of the disease (dates garnished with bat guano?).
At least Dr. Mackay has given us a day by which MERS will have to put up or shut up as a Hajj problem.