From Hellgatewiki.com

Theme Events

Subscribers get access to themed events throughout the year that go along with the time of the year. These events have special quests and items that go along with them. The items aren't any better than regular items but tend to have unique properties graphically or are items just for fun.

All Hallow's Eve

The first theme event of the game. Starting on official release day and running from October 31st, 2007 to November 4th, 2007. This event was opened up to everyone, because of issues FSS was having with people being able to subscribe. So instead of have lots of people who wanted to subscribe being forced to miss out on all the goodies of the event the first event was accessible to all.

Guy Fawkes Day

The second theme event of the game, set to run from November 5th, 2007 to November 11th, 2007. This event is only for subscribers and they will "Remember, remember the 5th of November".