RE: Can SCCM send a mass notifcation to all users?
As JSANDY mentioned, you could make a .VBS file using something like the following code, setting it only to run between specified times (so it doesn't pop up when you don't want it to).
'DISCLAIMER: This script is provided "as is" with all faults. The author cannot be held liable for any indirect, special, incidental, consquential, or exemplary damages arising out of or in any way relating to the use of this script, including without limitation damages for loss of goodwill, work stoppage, lost profits, loss of data, and computer failure or malfunction. You bear the entire risk as to the quality and performance of this script.
'Microsoft SCCM 2007 Notes and Comments: Configure the package Program so that it is set to install "Only when a user is logged on".
On Error Resume Next
Set objWshShell = WScript.CreateObject("WScript.Shell")
If Day(Date) = 27 Then
If Hour(Time) < 24 Then
objWshShell.Popup "Data Migration Starting at 10:00 P.M. Tonight!" + (Chr(13)& Chr(13)) + "In order to ensure that you do not lose anything, we ask you to please shut down your machine for the remainder of the night; as you may be forcefully disconnected (potentially losing any unsaved data)." + (Chr(13)& Chr(13)) + "We are sorry for any inconvenience," + (Chr(13)) + "Your friendly neighborhood techie" + (Chr(13)) + "email@example.com", 300, "IMPORTANT: Please Read Carefully!", 0 + 48