With Diamond and Pearl players were introduced to the first 'easter egg' in a PokÃ©mon game that would only occur on a specific date. In Platinum another date dependent 'easter egg' was introduced and it has yet to be seen if the birthday event from D/P was left in Platinum's encoding, or if, possibly, another birthday event has been embedded.
What is an 'easter egg' you ask? Well, if you did not already know, easter eggs are essentially hidden text, events, glitches, images, or sounds that can be found using various methods of input or by other means. The term easter egg was coined as finding the easter egg is similar to an easter egg hunt; go out and look for a hidden prize! The easter eggs found in these games are somewhat corny but hey, they still have a mild, momentary, fun-factor...
Below I have documented the relevant easter eggs, that I am aware of, that occur in Diamond/Pearl and Platinum. The easter eggs found here are generalized, which is to say I have not discriminated on their being only date dependent but rather any easter egg. If you think that you have an easter egg not documented here please contact me!
WiFi Plaza Mew
Dates: Monday, November 17 - Wednesday, December 17
Platinum's WiFi Plaza is transformed from the standard theme featuring the starter types of fire, grass, water as well as electric (lest we not forget Yellow versions Pikachu...) to a theme featuring Mew. The reason for the event to occur during these dates is unknown... I did some research to see if there was an explanation but the only things I could find, as they relate to this timeline are the "7-5-3" Festival (November 15) and/or the Labour Thanksgiving Day (November 23). If you have something solid let us know here.
Wild Foreigner Appeared!
In Veilstone City, in Japanese versions of Platinum, you will find this lonely man... When you talk to him he will speak to you in english text instead of the traditional hiragana.
Date: Saturday, January 12
Junichi Masuda is the game director of GAME FREAK, inc. and has also done some composing for the games. He celebrates his birthday on January 12th and as result an easter egg was dropped in so that "Diamond Dust" would fall from the sky, instead of traditional snow, in Snowpoint City, on that day. The Diamond Dust was basically reminiscient of the shape of a diamond and sparkled as if looking at a diamond. The event was hinted at on his website days before it occured and the ambiguous description lead many to get their hopes. As a result the event, to some, ended up being a disappointment but to others still retained some degree of a momentary fun-factor.