Not yet as a finished feature. Mailspring includes a Calendar Preview that you can open from the Developer menu. It shows the calendars of connected Google accounts, read-only, and is there for early feedback rather than daily use. Outlook, iCloud, and CalDAV calendars aren't shown in the preview.

Invitations

When someone sends you a meeting invitation, Mailspring shows the event details at the top of the message with Accept, Maybe, and Decline buttons. Choosing one sends your response to the organiser, and the event appears in your provider's calendar (Google, Microsoft 365, and so on) as it normally would. You'll also see when an organiser cancels or reschedules.

To add the event to a calendar that isn't connected to Mailspring, open the attached .ics file with your calendar app.