For most of the year, you do not strictly need to buy Shinkansen tickets in advance, as trains on the popular Tokaido line (Tokyo–Kyoto–Osaka) run as frequently as every few minutes. You can usually walk up to a kiosk or ticket window and board the next available train within 15 to 20 minutes. However, in 2026, there are three major "No-Go" periods where advance booking is mandatory: Golden Week (late April/early May), Obon (mid-August), and New Year's. During these holidays, trains sell out weeks in advance. Additionally, if you are traveling with "Oversized Baggage" (suitcases with total dimensions over 160cm), you must reserve a specific seat with a baggage area in advance, as these spots are limited and can be fully booked even on non-holiday days. Using the Smart-EX app allows you to book up to a month in advance and "tap-in" with your Suica or IC card, which is the most efficient peer-to-peer method for securing a reserved seat and ensuring your group sits together during the 2.5-hour journey.