f you are an Office 365 Company Administrator, you will receive an email notification once your tenant has been scheduled for a service upgrade. This email is typically sent several weeks before the upgrade is scheduled to take place.
In case you have missed this email notification, you can utilize the Windows Azure Active Directory Module for Windows PowerShell to detect if your tenant has been scheduled for upgrade, and also detect if your tenant has already been upgraded.
Note: This method does not show the exact date of the scheduled service upgrade.
Read more at source: http://blogs.technet.com/b/lystavlen/archive/2013/07/10/office-365-using-powershell-to-detect-service-upgrade-status.aspx