Provided you have an SMS available in the Document Manager against the Adult, Child and Group Postponed Letter Option you can use the Cancel Appointments in the Closures and Holidays in the Planner.
Go to 'Closures and Holidays'
Select the day you wish to postpone all the appointments,
Select 'Cancel Appointments' and tick the box Show Before Printing Letters
Highlight all the Patients then click 'Print Letters' this will create the Postponed Letter/SMS/email for each patient depending on what is their preferred communication.
Then I suggest you select 'Print List' this will give you a list of all the patients you have contacted.
Finally select 'Cancel Appointments' and this will mark all the appointments as Postponed.
I hope this answers your query, if not then please get back to us.