Jothar, welcome to the April Fools Day prank! There is NO such thing as a “YEARLY INTERNET MAINTENANCE” it is a prank done around this time of year because of April 1st.
There is no need to:
1. Disconnect all terminals and LANs from the Internet.
2. Disconnect all Internet servers from the Internet.
3. Refrain from connecting any computer, or any other Internet connection device, to the Internet in any way.
Enjoy your Chol HaMoed typing! (is that muttar? is it considered writing???)