Dead of Winter — ruling 1

The value of X is determined only as Dead of Winter begins to resolve. It won’t change later in the turn if the number of snow permanents you control changes.

Dead of Winter — ruling 2

Dead of Winter affects only creatures that aren’t snow creatures at the time it resolves. Creatures that enter the battlefield later in the turn won’t get -X/-X.

Dead of Winter : règles, fonctionnement et interactions (MTG)

Dead of Winter 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

All nonsnow creatures get -X/-X until end of turn, where X is the number of snow permanents you control.

How it works : Dead of Winter

Original English text — Wizards of the Coast / Scryfall

#1June 14, 2019· Wizards of the Coast
The value of X is determined only as Dead of Winter begins to resolve. It won’t change later in the turn if the number of snow permanents you control changes.
No other card is named in this ruling.
#2June 14, 2019· Wizards of the Coast
Dead of Winter affects only creatures that aren’t snow creatures at the time it resolves. Creatures that enter the battlefield later in the turn won’t get -X/-X.
No other card is named in this ruling.