Houseguests rarely cause a sewer backup by themselves; they expose a main line that was already partly blocked. Extra showers, flushes, laundry and dishwasher loads send more water down the line at once. A pipe narrowed by roots, scale, grease or a sag handles a normal day but backs up under the heavier flow.
The restriction decides how much water the line can pass. Roots grow into joints, cast iron in older homes scales from the inside, and a belly in the sewer line holds waste in a low spot. With a few people at home, water trickles past. With a full house, it arrives faster than it can leave and rises out of the lowest opening, usually a tub or shower, which is why sewage in the bathtub is often the first sign. Guests also flush wipes, paper towels and hygiene products that snag on that same restriction.
Before guests arrive, spread laundry and dishwasher loads across the day instead of running them during the morning shower rush, and put a lidded trash can in the guest bath so only toilet paper goes in the toilet. Find the sewer cleanout outside and keep it reachable, since that is where a plumber clears the main line. If drains were already slow or gurgling, have the line cleared or get a sewer camera inspection before the holiday rather than during it.
One honest limit: clearing a clog before guests come buys time, but it does not fix a belly, a crack or a broken section. Those collect waste again and the backups return. Only a camera shows which problem the line has, and whether cleaning is enough or the pipe itself needs repair. A week-before checklist is in these plumbing tips for Thanksgiving.