How do you plan for Boxing Day + Black Friday peak load?

UK retail has two peak weekends, not one, Black Friday (~£8bn weekend) and Boxing Day (post-Christmas luxury blowout, huge for Mayfair / Knightsbridge brands). We build a war-room recipe: (1) synthetic load test at 50× baseline using k6 or JMeter, two weeks before peak; (2) Redis cluster + Varnish FPC with smart-purge instead of full flush; (3) queue-isolated checkout, checkout traffic on its own PHP-FPM pool so a catalogue spike can't starve add-to-cart; (4) Adyen tokenisation for repeat buyers so card-form latency drops 80%; (5) read-replica DB for product browsing, primary DB only on order placement; (6) Fastly or Cloudflare in front of Magento for static + image cache. Wimbledon (June-July) and pre-Brexit-tax-year (April-May) are smaller peaks we also plan for.