You don't need to purchase shinkansen tickets in advance unless...
Many people on this subreddit recommend not buying shinkansen tickets in advance. While that is generally good advice, I wanted to highlight some situations where you should get them early.
- Golden week or other Japanese holidays and big events. Golden Week is April 29 - May 5 but also be aware if it starts on a Monday or Tuesday or ends on a Thursday or Friday because that can extend the length of vacation time. Dec 29-Jan 3 will also see a large number of travelers. The third busiest travel time is obon season in mid-August. If you are heading to an area with a big festival or during sakura season, that can also affect availability.
- Green car or GranClass. We will be going from Sendai to Akita on 4/19. That's 27 days away. There are currently only 4 seats left and no adjacent ones in the green car. It's also a Saturday and the beginning of Sendai's sakura season. It is a good thing I booked it 2 days ago.
- You have a large group - like a family of 4 - and you want to be sure you are sitting near each other.
- You want very specific seats such as a window view of Mt. Fuji. [Added]
- You have oversized luggage - when L+W+H > 160 cm. [Added]
Do NOT book ahead of time if you are flying in and then taking the shinkansen to another city. That is just asking for trouble if your flight is delayed or it takes longer than expected to get through passport control.
I hope people find this helpful! Have a fun time in Japan.