The days before a seasonal sales peak are usually when a store's image setup changes the most: new promotional banners on the homepage, a category banner for the sale itself, sometimes an entirely new landing page with images that were never uploaded before. Each of those placements needs to be covered by a preset (category_banner, or a custom one) with the right dimensions and resize mode; anything dropped into a page without going through the ImagePipe Gutenberg block or an existing preset placement won't be picked up by automatic replacement, which is expected behavior rather than a bug, but worth checking for on new pages built in a hurry.
Once presets are confirmed, the next practical step is warming the cache before traffic arrives rather than after. Since each variant (a given file, size, mode, and format) is only generated the first time it's requested and then served from the permanent disk cache afterward, the very first visitor to a page after a preset or image change is the one who pays the generation cost. Visiting the key pages themselves, homepage, main category pages, and the top product sheets, a few hours before the peak generates and caches those variants ahead of time, so real visitors during the peak only ever hit the cache.
For a store running several seasonal storefronts, or an agency managing more than one client site, it's also worth checking the site quota at /portal/licenses before the campaign starts, particularly if a temporary or additional site is being spun up just for the sale, so an activation slot is available in advance rather than discovered missing on the day itself.