In a Google to Google migration, where the primary source domain is being moved to a Google Workspace destination, calendars should be migrated before the source domain is deleted. Users' secondary calendars are deleted and cannot be recovered 'if the original primary domain (which becomes a secondary domain following the rename) is deleted'.
As migrated calendar events cannot be changed/updated, it is recommended to carry out a full calendar migration shortly before carrying out the domain switch to ensure you migrate the most recent version of users' primary and/or secondary calendars.