Use Ctrl or Cmd ⌘ + Click to select multiple events. Selection is maintained throughout calendar navigation, so you can select events in a particular month or week, navigate forward, and select again to add to your selection.
Use Ctrl or Cmd ⌘ + A to select all events in a given view. This shortcut also respects filters, so you can quickly select everything with a particular tag by filtering by that tag and selecting all.
To edit the details of multiple events at once, select the events you want to edit and click Edit or use hotkey Ctrl/Cmd ⌘ + E.