Lead with the new time
Put the new date, time and time zone in the first line.
Say why in one sentence
Give the real reason, such as an instructor illness or a venue change.
Offer options
Say whether there is a recording, a make-up session or a spot in another cohort.
Send it everywhere
Email, community and calendar invites should all say the same thing.
Answer the replies fast
Expect a wave of questions in the first hour and use a saved reply.
Confirm attendance
Ask students who cannot join to reply so you can plan.