Let's start by talking about what festivals are like in Japan.

Festivals are generally largely secular events, held at various times throughout the year but usually most numerous during summer, that people of all ages attend for fun and a good time. While many of the festivals are a backdrop to various religious services, such as Shinto and Buddhist rituals, it's not comparable to going to Christian mass with a party afterwards.

Many of the people watching are not actively 'worshiping' or practicing their faiths (or any faiths) while in attendance at the festival. For most people that go to these events, it's just about eating good food, playing festival games, and having a good time with friends and family. It cannot be overstated how nonreligious these events are for the lay person.

The most comparable thing is a carnival or maybe even when a circus comes to town (minus all the animals). For smaller towns, these festivals are usually the event of the season and everyone goes. Festivals in small town Japan are what sports games are to small town America.

Festivals are also very local events. Even festivals that celebrate the same thing can differ widely from area to area, town to town. This is especially true for Shinto festivals. Different areas have different kami that they revere (since kami are necessarily local) and traditions can be vastly different.

The Mitama Matsuri is a festival held at the Yasukuni Shrine that celebrates the national holiday of Obon. Obon in general is a holiday that celebrates one's own ancestors as well as thanks-giving to all ancestors of Japan.

The Mitama Matsuri celebrates Obon with a focus on celebrating the ancestors who did great service for Japan, because Yasukuni Shrine celebrates those who died in service of Japan since the Meiji Era. While many of those enshrined are military, there are also many civilians who received the honor of being enshrined there.

The Mitama Matsuri doesn't have any significant connection to samurai in particular. They would be included under the heading of 'celebrating and revering our ancestors' but that goes for everyone else as well.

In general, it's important to stress how unpolitical it is for Japanese people to attend the festival.

For them, it's just a party. The vast majority of people who visit the shrine during regular times attach no special meaning, political or otherwise, to their visit. This goes double when there's a festival happening. People go there for food, drinks, and fun.

That being said, the private organization that runs Yasukuni Shrine is pretty unabashed about their historical revisionism.

But it should also be stated that it's the views of a private organization and inherently not representative of the Japanese people at large. Regardless, these views never come into play at the Mitama Festival, which is readily apparent for anyone who visits the event themselves, so no need to take my word for it! It's also beautiful and fun to boot!