While Shinkansen (Bullet Train) tickets rarely "sell out" in the sense that you are stranded, reserved seats and specific time slots absolutely sell out during peak Japanese holidays. In 2026, the "Golden Week" period (late April to early May), Obon (August), and the New Year season see massive surges in domestic travel. During these times, if you don't book in advance via the Smart-EX app or at a station, you may be forced to stand in the "Non-Reserved" (Jiyu-seki) carriages, which can be extremely crowded and uncomfortable for long journeys. Furthermore, the Nozomi trains—the fastest on the Tokaido line—often switch to "All-Reserved" seating during these peak periods, meaning you cannot board at all without a pre-purchased seat. For standard travel days, you can usually buy a ticket 15 minutes before departure, but for any major holiday, booking at least 30 days in advance is highly recommended.