How to use summary field conditionally for display?
The Description text field needs to be of type Long text and summary.

With view modes (admin/structure/types/manage/band/display) you can configure how the fields must be rendered.

In the view mode Teaser you could set the format to Summary or trimmed and in other view modes to Default.

When viewing the content type Band as a full page you can render the view mode Default. When rendering a list of Bands with views you can set the view mode to Teaser.