Egg Hatching In Generation 4
When an egg is being created, the game figures out which Pokémon will hatch from it and how many cycles are required. An egg cycle is just a counter, usually set to 255, that ticks down with every step you take. The amount of egg cycles an egg needs to hatch varies from Pokémon to Pokémon. In Generation 4 games your egg cycle count starts at 0, so a Magikarp egg would actually take 6 cycles to hatch instead of 5.
Once you take enough steps to finish an egg cycle, the remaining cycles for all eggs in your party decrease by 1. Once you go through all cycles for an egg, the egg hatches. One caveat here is that if an egg completes all of its cycles, and there are other eggs in your party, those other eggs do not have their current cycle processed. This means if you have 5 Magikarp eggs in your party and tried to hatch them all at once, your first egg would hatch in 6 cycles, the second would take 7 cycles, and so on.
You can speed up egg hatching by using a Pokémon that has the Flame Body or Magma Armor ability. If either of these abilities are present in your party, every 255 steps would reduce your egg cycles by 2 instead of 1, effectively doubling your hatch speed.
This cycle step counter isn’t tied to any specific egg. This counter is running in the background even when you don’t have an egg. This is hard to notice because every time you receive an egg your step counter gets set to 0, so your egg will hatch when expected. This means that if you are halfway through one egg’s cycle and receive a new egg, that half of a cycle is lost forever.
Eggs also process in party order. This means that if the first egg in your party hatches, all other eggs in your party won't have their cycle updated until the next 255 or 230 steps have been taken. So optimally while hatching you should stagger your eggs so they don't all have the same amount of cycles remaining, and reorder them in terms of cycles remaining from highest to lowest.
Special Dates
In Pokémon Diamond, Pearl, Platinum, HeartGold, SoulSilver, Brilliant Diamond, and Shining Pearl, there exists special dates that can impact your encounter rates and egg hatching speed. On these special egg hatching dates, the egg cycle step count required goes down to 230 from the normal 255. Here are all dates that impact your egg hatching speed:
| Date | DP/BDSP | Platinum | HGSS |
|---|---|---|---|
| January 12th | ✅ | ✅ | ✅ |
| February 14th | ✅ | ✅ | ✅ |
| March 3rd | ✅ | ✅ | ✅ |
| April 1st | ✅ | ✅ | ✅ |
| May 1st | ✅ | ✅ | ✅ |
| June 11th | ✅ | ✅ | ✅ |
| July 7th | ✅ | ✅ | ✅ |
| August 21st | ✅ | ✅ | ✅ |
| September 7th | 🚫 | ✅ | ✅ |
| September 28th | ✅ | ✅ | ✅ |
| October 31st | ✅ | 🚫 | ✅ |
| November 21st | ✅ | ✅ | ✅ |
| December 14th | 🚫 | ✅ | ✅ |
| December 24th | ✅ | 🚫 | ✅ |
| December 25th | ✅ | 🚫 | ✅ |
This mechanic has time travel protections, meaning that if you try to modify your system date to go straight to one of these dates, the step decrease won't activate. Instead, you must change the date to one day before one of these dates, save in game, then wait approximately 24 hours.