Midnight exploit

The midnight exploit in Pokémon Diamond/Pearl/Platinum, Pokémon HeartGold and SoulSilver and Pokémon Black and White allows certain daily events to re-occur.

It can be performed simply by changing the date on the Nintendo DS or Nintendo 3DS system, and the player does not have to wait a full day.

It involves changing the clock on the Nintendo DS or 3DS firmware to a time near midnight, playing the game and waiting for the time to become midnight in the game. The in game time can be checked by having the Pokétch set to the Digital Clock application.

Changing the day of the week through the Nintendo DS or 3DS firmware normally does not work, but the Seal Market goods list in Sunyshore City (at least in Pokémon Platinum) can be changed this way.

Some, if not all of the 'midnight exploitable' events will not re-occur by waiting for the clock to turn midnight on the Nintendo DS firmware. The player may have to actually be in game.

Procedure

Set the Nintendo DS clock back to a time such as 1 minute before midnight.
Save the clock data and shut the Nintendo DS off.
Start the game, wait until midnight (this can be checked with the Digital Watch Pokétch app).

Changeable events in Pokémon Diamond, Pearl and Platinum

For more information about these events, see the relevant Serebii pages (Diamond and Pearl, Platinum)

Random Pokémon Center battles (Pokémon Platinum)
Swarm Pokémon (viewable in the opposite gender rival's house in Sandgem Town)
Lottery (Jubilife City)
Floaroma Town Berry gift
Route 208 Berry gift
Pokémon Reporter (Solaceon Town)
Massage woman (Veilstone City)
Game Corner hidden coins (EU Platinum)
Pastoria City Berry gift
Great Marsh shard gift (Pastoria City)
Pokémon Mansion owner's cute Pokémon experience. Finding out what Pokémon it is requires leaving the room and re-entering if the butler is not present (Route 212).
Pokémon Mansion 5-Maid Knockout Exact-Turn Attack Challenge (requires leaving and re-entering the room to try again)
Celestic Town Pokémon Center Great Ball man (requires a Pokémon with high happiness to receive the Great Ball).
Snowpoint Town trendy saying.
Seal Market goods (Sunyshore City, also changeable by changing the Nintendo DS date).
Julia's Ribbons.
Belt man (Route 221).
Ribbon Syndicate spa (only tested in Platinum, requires at least 10 Ribbons spread among current Pokémon party to enter the Ribbon Syndicate. Requires leaving and returning to the room if the staff are close together).
Top Trainer Café (requires leaving and entering again).
Underground goods that traders have.

Changeable events in Pokémon HeartGold/SoulSilver

For more information about daily and weekly events in Pokémon HeartGold and SoulSilver, see this page on Serebii.

Swarm Pokémon.
Goldenrod Radio Tower lottery.
Olivine City Seal gift.
Route 16 trendy saying.

Certain events that are consistent on certain days of the week (e.g. the Pokéathlon Dome prizes) can be changed with both the midnight exploit and by manually changing the date on the Nintendo DS firmware.