Party Crasher — ruling 1

During each combat, there’s a time when attackers are declared. Normally, only the active player declares attackers, but Party Crasher doesn’t play by those rules. During the declare attackers step of your opponent’s combat phase, after they’re done declaring their attackers, you may choose to declare Party Crasher as an attacker. After you declare blockers, the defending player can declare blockers for Party Crasher. All combat damage is dealt simultaneously, unless a strike ability (first, double, last, and/or triple) is involved.

Party Crasher — ruling 2

If Party Crasher attacks during an opponent’s turn, that opponent could block it with an attacking creature that is untapped as blockers are declared. That creature will then be attacking and blocking at the same time. If Party Crasher is also untapped as blockers are declared, it can block the attacking creature that blocked it. Even if all this happens, the creatures will only deal combat damage to each other once.

Party Crasher : règles, fonctionnement et interactions (MTG)

Party Crasher is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Haste You can attack with this creature once each combat during each opponent's turn.

How it works : Party Crasher

Original English text — Wizards of the Coast / Scryfall

#1January 19, 2018· Wizards of the Coast
During each combat, there’s a time when attackers are declared. Normally, only the active player declares attackers, but Party Crasher doesn’t play by those rules. During the declare attackers step of your opponent’s combat phase, after they’re done declaring their attackers, you may choose to declare Party Crasher as an attacker. After you declare blockers, the defending player can declare blockers for Party Crasher. All combat damage is dealt simultaneously, unless a strike ability (first, double, last, and/or triple) is involved.
No other card is named in this ruling.
#2January 19, 2018· Wizards of the Coast
If Party Crasher attacks during an opponent’s turn, that opponent could block it with an attacking creature that is untapped as blockers are declared. That creature will then be attacking and blocking at the same time. If Party Crasher is also untapped as blockers are declared, it can block the attacking creature that blocked it. Even if all this happens, the creatures will only deal combat damage to each other once.
No other card is named in this ruling.