Does YOUR country have a gathering in one place of hundreds of thousands of people each year? If so was it cancelled?

Hundreds of thousands, no (this is a small county), just 91,000 or so.  And yes, it was canceled.

Our annual county fair draws a ‘fairly’ large crowd (pardon the pun).  It was scheduled to run from August 12 through 16.  However, due to the virus it was canceled.  (and many people are none too happy)

We also have numerous other large crowd events (i.e. music festivals, livestock events) that have either been canceled or postponed as well.  The s***’s really hit the fan if they have to cancel our famous Christmastime events. 

No, not hundred of thousands all in one go. (In the UK)]

Whet we do have is a huge number and range of public gatherings from major professional sports and music events outdoors or in big stadia, via all manner of special-interest pro and amateur-run events like vintage-vehicle rallies, large exhibitions, town carnivals and Christmas street-fairs, to village church fetes run by local volunteers.

Some of the big ones host thousands at once, perhaps on each of a few days; a village fete sees maybe a couple of hundred people over the afternoon.

However, if you are asking against the pandemic context, we need consider that many people who attend one event might attend others of the same type or entirely different.

All in the present tense.....

A major exhibition I had hoped to attend in October is now off. The organisers had waited as long as possible before deciding, but it is now clear that basically all large public events are likely to be write-offs for the rest of the year. 

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There is a pair of otherwise-unique public annual events run by volunteers in the North of England, that were both cancelled this year. These are something called the Gaping Gill Winch Meet, run one each by two local caving-clubs. These set up diesel-hydraulic winches by which paying tourists can be lowered down the 340ft deep Gaping Gill pothole, to view the huge chamber below. (Oodles of photos on-line!) Normally you can only see it if an experienced, fully-equipped caver. Now, these events are outdoors, way up on an exposed fell. Safe enough? Ah, no. It brings a lot of people into a relatively small area; but also,certain parts of the operation entail the volunteer operators being very close to each visitor, making any pandemic precautions problematical to say the least.

They also raise a wider point in that these and a lot of other public events in rural areas bring a lot of visitors to the villages, but a result of easing the lock-down has been huge influxes of people to very many beaches, inland beauty-spots etc., to the point where the locals just do not want them.  Even when car-parks and public toilets had been re-opened, a lot of the visitors have made themselves utterly unwanted by having no respect for their surroundings, other people - nor evidently for themselves. So they block roads and farm gates with inconsiderate parking, abandon vast amounts of litter and even human faeces, cause heath fires with disposable barbeques, abuse and spit at anyone trying to instil some sort of order, and so on. 

My own village has two Christmas-tide events: an afternoon street-fair, and public carol-singing, both in the Square and tending to become quite crowded. They are in December but I will be surprised if they go ahead this year.

So for the time being, the restrictions and their effects go a lot further than the obvious.

The biggest is the Australian Football League Grand Final - analogous to your Super Bowl, but our footballers play a much faster game and don't wear armour like a bunch of pansies.
It hasn't been cancelled yet, but it seems certain that if it does go ahead it won't be at the Melbourne Cricket Ground, where it has been held for 100 years, due to Victoria being locked down. Three alternative venues are being considered, none with the capacity of the MCG but in cities that are relatively COVID free - the Adelaide Oval, the Woolloongabba Ground in Brisbane or Optus Stadium in Perth.