For a Seoul–Gyeongju trip, count the local transfer after the KTX as part of the journey. Singyeongju Station is outside the historic centre; write down the first stop and how you will reach it before deciding that a day trip fits.
When a day trip fits
A day trip works best when one or two connected places are the priority and the return train still leaves enough time to get back to the station. Do not plan an overnight-sized route and then cut every stop to catch the last service. The first local connection, luggage and any wait for check-in or storage all reduce the usable day.
What the overnight adds
One night makes sense when you want dinner in Gyeongju, an evening walk or a first stop the next morning. If the only goal is one site, spending the night may add another hotel move without adding much time at that site. Compare the overnight itinerary against the day-trip route before reserving a room.
Choose the Seoul date and return plan
Borrow a complete day from the Seoul schedule, not the arrival or departure day and not a date with a hard-to-move booking. After the return, leave the next Seoul morning workable. For an overnight, compare Gyeongju hotels only after you know which area you need.
Check the current KORAIL timetable and ticket rules, the first and last local transfers, luggage storage and the next morning’s opening time. Trains, buses and local operating conditions change; recheck them before departure.