The "4 Year Anniversary Calendar Wallpapers" can be found at the bottom of the Wallpapers page.
They did list the events, but only ranged from May 2008 to April 2009. Many people requested CoH calendars after that, so I created some unoffical ones from May 2009 to September 2010 (found here), but I didn't list any official events on them, and I haven't seen any other offical calendar since then either.
Best thing to do is stay tuned to the Forums for announcements. I'd say to make sure your account E-mail address is up to date and sign up for the e-mail alerts, but I think those are handled by a third-party, and often arrive much later than forum announcements. (Even if you don't sign up for alerts, keep your account E-mail address is up to date anyway though 
)
The "Official" calanders were great. I never wondered about DXP weekends. Really helpful in planning random weekends off from work.
Le Blanc 50 Dark/Dark Scrap
High Huntress 50 Archery/NRG Blast
And a goatload of others. On a goatload of servers.
Official Rickroller of Hero Con 1
I recall that there used to be a calendar for official events like rikti invasions, double xp weekends, etc. Does this still exist, and if so where? I have only been able to find the player events calendar.