It's always important to notify your userbase when a system outage is going to occur. For corporate IT departments, the best method is usually email. Here is a template that does a great job covering the bases:
Summary
What? {one sentence summary}
When? {date and time}
Who? {applicable parties}
Details
{Give a detailed and friendly description of what's occuring. Explain why this downtime is happening, when it is happening and how long the outage is expected to last.}
Services impacted: {List the services or systems impacted.}
What is not impacted? {List adjacent services one might concude may be affected but should not. This gives your users a rubric for systems that should not go down. Should those systems go down, it is a different issue, and the user can submit a support ticket.}
Impact: {Tell the user what to expect during the downtime. Again, this is a rubric to guide the users in determining if they are experiencing the expected outage, or a different issue. Reiterate the expect length of downtime.}
Questions? For more information, please contact {the support team name} by submitting a support ticket via {help desk ticketing link}, by emailing {support team email address}, or by calling {support phone number}.
Emergency issues: In the case of extreme emergencies directly related to this change, you can email: {emergency support email address} or call {emergency hotline number}.