Once a year the cherry trees bloom and people gather to have a pic nic to appreciate the flowers. This is hanami.
In Matsumoto, the flowers bloomed last week, and sadly they are already almost gone.
However if you plan on visiting next year here are three locations to enjoy sakura in Matsumoto.

Metoba River

Walking along the river, near frog street, you can enjoy cherry blossoms. The riverside is also a great place to have a pic nic with friends in the afternoon.

Matsumoto Castle

Every year, as the flowers bloom, the castle is lighted up with lanterns and sometimes the inner garden is open to visit for free. This is a nice evening view. The lightup lasts until 9PM.

Koboyama

Koboyama is a small hill and a kofun (an old grave). The hill is covered in sakura, and it is a great early evening hanami spot. From the top of the hill you can enjoy a view of Matsumoto City.

It’s a 40 minute walk from the station, I recommend planning beforehand!

In reality, even without going to a specific place, you will be able to enjoy sakura a bit everywhere, all it takes is being in the right place at the right time! 
I hope you have a good hanami!