You can castle king-side (o-o) or queen-side (o-o-o), providing that neither the king, nor rook has moved, and also that the king is not moving thru check, out of check, or into check.
Can You Castle on Both Sides of the King?

If you can't remember how to place your kings, this rule applies: Move the king 2 squares to the rook, and then place the rook on the other side.
After 0-0 (Short-side castling) the king will always be on g1 (g8), while the rook on f1 (f8).
After 0-0-0 (longside castling) the king will always be on c1 (c8) and the rook on d1 (d8)

So the once each game means only one player a game can castle
lol
well i did it last game and my friend got super mad and i didn't know

Not quite difficult, though : when a pawn just queens between the king and the rook, then the king will take the new queen en passant while castling.

Can You Castle on Both Sides of the King or just the Side Closest to the Rook?
Heidenfeld castled on both sides, so why can't we?
"http://timkr.home.xs4all.nl/records/recordstxt.htm#greatest%20number%20of%20castlings"
(Paste the link into your browser without the quotes.)

Each player may castle once a game as long as all conditions allowing castling are met.
What aln67 said about en passant is just gibberish intended as a joke.

So the once each game means only one player a game can castle
No!
There are 2 players in a game of chess.
Both players have the option of castling 1 time during 1 game.
As long as the castling coniditions are met!
These conditions apply to both players.
Conditions:
- You can castle king-side (o-o) or queen-side (o-o-o) 1 time during the same game. (providing that neither the king, nor rook has moved.)
- The king can't castle moving thru check
Black's bishop on a6 is hitting the f1 square.
White can't castle here because if he was to castle his king would have to move through f1 to get to g8.
Since black is hitting f1 white would be moving thru the attacked square.
This is an example of white not being able to castle thru check.
- The king can't castle moving out of check
In this position black has the white king in check!
The white king can't castle here because he is currently in check.
What white will have to block the check in order to castle!
The move Be3 would be a way for white to block the check so that next move he can castle.
This is an example of white not being able to castle out of check.
- The king can't castle moving into check.
Black's bishop on c5 is hitting the g8 square.
White can't castle here because if he was to castle his king would end up on the g8 square.
The g8 square is currently being attacked.
Which means white would castle into check.
Which isn't allowed!
This is an example of white not being able to castle into check.
NOW IT IS TIME FOR MORTAL KOMBAT TO TEST YOUR WITS!
THEME SONG:
https://www.youtube.com/watch?v=EAwWPadFsOA
CAN WHITE CASTLE?
Problem 1:
TEST YOUR MIGHT!
Problem 2:
FIGHT!
Problem 3:
EXCELLENT!
Problem 4:
TEST YOUR MIGHT!
Problem 5:
MORTAL KOMBAT!
Problem 6:
FIGHT!
Problem 7:
EXCELLENT!
Problem 8:
TEST YOUR MIGHT!
Problem 9:
MORTAL KOMBAT!
Problem 10:
How many did you get right?
I will post the answer below in white text:
CAN WHITE CASTLE?
YES = P#1 - P#2 - P#5 - P#10
N0 = P#3 - P#4 - P#6 - P#7 - P#8 - P#9

I can castle on the inside of the king,
but I'm such an agoraphobic so I eat fatty foods,
and I pad the walls of my king from intruders,
but things get warm and stinky,
and when mucus begins dripping down my pinky,
I find it's almost time to flee from the looters,
but then I'd have to go outside my king, dudes...
so I'll just castle in here and hope my rook protects everything.

#13 of course it was a joke :-)
Your a joke.
Your question is a joke. Why are you here if you do not know the basic rules of chess? You should be embarrassed. Go to the local library and look at a basic rule book. Posting a question like this on a chess site is like asking a dietician if it's necessary to open your mouth in order to eat!
I'm sure chess.com is fine with having users who don't know all of the rules of chess when they first create their account. For the site it's just another potential customer.
Also, shouldn't someone be able to learn the rules of chess from a chess site?

Perhaps the guy is a young child, that makes the question more reasonable. But, a commentator made a joke and followed up by telling the OP "It's just a joke." The OP made a personal attack on the commentator by telling him, "you're a joke." Can you not see the difference?
Good Grief!?
The OP didn't attack anyone. His original comment was three years ago, and he last logged in 6 months ago.

#13 of course it was a joke :-)
Your a joke.
Your question is a joke. Why are you here if you do not know the basic rules of chess? You should be embarrassed. Go to the local library and look at a basic rule book. Posting a question like this on a chess site is like asking a dietician if it's necessary to open your mouth in order to eat!
I'm sure chess.com is fine with having users who don't know all of the rules of chess when they first create their account. For the site it's just another potential customer.
Also, shouldn't someone be able to learn the rules of chess from a chess site?
Sure, sure you are correct, it's about revenue. No one here has denied this guy his privileges, why do you want to deny mine? I have a right to state my opinion and, that is my opinion. You have stated yours and I accept that.
Perhaps the guy is a young child, that makes the question more reasonable. But, a commentator made a joke and followed up by telling the OP "It's just a joke." The OP made a personal attack on the commentator by telling him, "you're a joke." Can you not see the difference?
Good Grief!?
Where did I deny your privileges? Does disagreeing with you constitute a denial of your right to express your opinion?

#13 of course it was a joke :-)
Your a joke.
Your question is a joke. Why are you here if you do not know the basic rules of chess? You should be embarrassed. Go to the local library and look at a basic rule book. Posting a question like this on a chess site is like asking a dietician if it's necessary to open your mouth in order to eat!
I'm sure chess.com is fine with having users who don't know all of the rules of chess when they first create their account. For the site it's just another potential customer.
Also, shouldn't someone be able to learn the rules of chess from a chess site?
Sure, sure you are correct, it's about revenue. No one here has denied this guy his privileges, why do you want to deny mine? I have a right to state my opinion and, that is my opinion. You have stated yours and I accept that.
Perhaps the guy is a young child, that makes the question more reasonable. But, a commentator made a joke and followed up by telling the OP "It's just a joke." The OP made a personal attack on the commentator by telling him, "you're a joke." Can you not see the difference?
Good Grief!?
Absolutely not! But, you are now deliberately sidestepping my main point!
I'm sidestepping it because it was already addressed by the poster above me.
And, if you're going to make a claim like that do you really expect me to not address it?
Can You Castle on Both Sides of the King or just the Side Closest to the Rook?