Your email will come as a shock and a surprise to all employees. When this happens, employees get worried and rumors start. Therefore, your email must be precise and must give assurance that all is well (unless, of course, it is not!).
Memo to ALL Employees:
The company is undertaking some important activity, which requires your presence. Therefore, please note that from (insert start date) to (insert end date), employees may not take leave. In exceptional circumstances and with written approval from management, leave can be granted. The activity comprises (insert what is happening). If you are able to provide this information; if you cannot, then:
"Details of the activity will be given shortly."
Thank you for your cooperation. On a personal note, may I, on behalf of Management, apologize for any inconvenience caused while we undertake this activity.
Name Designation
I hope the above helps.
Regards,
Harsh