The story
There is no traffic pattern in WordPress sharper than a ticket on-sale. A venue can spend a month at a few hundred visits a day and then take a year’s worth of concurrent load in a single minute, all of it pointed at two pages.
What makes it hard is that those two pages are opposite in kind. The event page — line-up, times, seating plan, directions — is completely static and identical for everyone. The checkout is per-person, stateful, and must never be cached at all.
Treating them the same is what breaks on-sale day. Serving the event page as prewarmed static HTML takes almost all of the load off the server, which leaves the capacity that remains for the checkout that genuinely needs it.