Legendaries.

I will tell you about the Kanto legendaries.

As you already know, Zapdos is in the powerplant, so no worries there. But, they say that he has the power to drain the energy out of your batteries if you fight him long enough, so be careful.

Articuno does not appear anywhere. Articuno is always there, has always been there, and will stay there until you catch him/her/it. All you have to do to catch this Legendary Pokemon is push every boulder you see into the near-by hole by using strength. After that, drop down into one of the holes and surf around till you find him. If for some reason you start getting pushed downwards, then you know that you missed a few rocks.

Moltres is at the top of the Ember Mountain on Island One, bring plenty of water Pokemon, and burn heals. If for some VERY strange reason Moltres is not there, then I suggest that you go look in Victory Road, the Elite Four cave. He will be burning the roof off of one of those places.

Mewtwo is in the special cave in Cerulean City, you must surf like you are heading towards the Power Plant, but turn left instead. He will be in the cave, which I will tell you, is Confusing. All the Pokemon in there are on high levels, with Mewtwo leading the pack at a whopping level 70.

Oh, You can also get Either Suicune, Raikou, or Entei depending on the Starter Pokemon that you chose at the beginning of the game. Charmander you can get Suicune, Squirtle you can get Raikou, and if you chose Bulbasaur you can get Entei!

To get all the other Legendaries from Emerald, and the like, you can use an Action Replay, or trade with some who has them.

What do you mean, "when" he appears? Articuno is always there, you've just got to solve the puzzle of the cave-push boulders to stop the current in the basement, then you'll be able to Surf to Articuno.

Moltres is at the peak of Mt. Ember, northeast of One Island.

Mewtwo is in Cerulean Cave, but you'll have to defeat the Elite Four first to open it; careful-it's at level 70 and knows attacks that will give you a run for your money: Recover, Psychic, Swift, and I've forgotten the fourth.

Suicune/Raikou/Entei: After you defeat the Elite Four, one of them will be around in Kanto. What one appears depends on your starter: Bulbasaur-Entei; Charmander-Suicune; Squirtle-Raikou.

Lugia and Ho-Oh, I believe, can only be reached with a special pass that's obtainable by cheating or from a Pokemon event. (other users-please correct me if I'm wrong; I may be thinking of Pokemon Emerald)

I don't know about Celebi, though.

1st Moltres is at the Mt.Ember Must Have the TRI Pass To Go The Island 1.. Use Surf To Mt. Ember .. Well, There Is A House Near The water Go There Use Surf And Keep Moving Up And Find Your Way To The Statue And Press A It Will Battle With You.. Can Catch Also.

Fushia City The Pkm Centre There U Keep Moving Down Then Use Surf... Then Battle Trainers Already There U Must Go Down Then Turn To Left.. U Go Inside Must Push Boulders To Each Hole.. Just 2 Boulders Will Do And U Drop Down U Are Already Surfing Keep Going In Front( Dun Go To the Dry And Keep Moving U See Aticuno Statue Press A It Will Battle..
Can Also Catch It

Zapdos At Power Plant