For some holidays, Linden Lab has changed the texture of the sun and/or moon to something fitting for that holiday. This tradition started in 2003 either for Halloween or Valentine's Day.
Friday The 13th Moon, Jason's mask from the popular horror movies, picture by Loki Pico
Not considered a holiday by most, but the day gets its own custom moon with the image of Jason's mask from the popular Friday The 13th movies.
Halloween Moon, taken in 2005 by Missy Martin
Halloween night moon above Freelon, scary "man in the moon" face. Taken in 2003.
An eerie image cast down on the world in the form of a jack-o-lantern type figure in the moon.
St. Patrick's Day sun, a large clover in the sky.
Sometimes only the sun is altered as opposed to just the moon or both. On St. Patrick's day a bit of luck is spread across the land by the light of a giant clover.
Valentine's Day moon, a silhouette of a couple kissing.
Valentine's Day sun, giant heart with an arrow through it.
The romantic holiday of Valentine's Day is brought out more through these alterations. During the day a shining bright heart hit with Cupid's arrow can be seen and during the night a more romantic imagery is given with the silhouette of a couple kissing.
Linden Labs also changed the sun on Martin Luther King day. Thanks to AJ DaSilva for the picture.
Martin Luther King Day, observed in the United States on observed on the third Monday of January each year, around the time of King's birthday, brought about a texture featuring the image of the slain civil rights leader.
2006 Easter Egg Sun.
During the Easter holidays (at least 2006) the sun was changed to be a colorfully painted egg in the sky.
Million Member Sun
Million Member Moon
In honor of Second Life gainng its 1,000,000th account on October 18th, 2006, the sun and moon texture were augmented with the Second Life logo and the number "1,000,000" below.
With the exception of Martin Luther King day, 2007, there have been no holiday suns or moon since the million member variety. It is likely this will go away entirely with the launch of viewers with WindLight based skies.