Locket of Yesterdays — ruling 1

Locket of Yesterdays reduces only the amount of mana you need to pay to cast a spell. The spell’s mana cost and mana value are not affected.

Locket of Yesterdays — ruling 2

Colored costs can’t be reduced by this effect. For example, a spell that costs {2}{R} can’t be reduced by more than {2}.

Locket of Yesterdays — ruling 3

The spell being cast doesn’t reduce its own cost, even if you’re casting it from a graveyard. At the time the cost is reduced, the spell is on the stack.

Locket of Yesterdays : règles, fonctionnement et interactions (MTG)

Locket of Yesterdays 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

Spells you cast cost less to cast for each card with the same name as that spell in your graveyard.

How it works : Locket of Yesterdays

Original English text — Wizards of the Coast / Scryfall

#1September 25, 2006· Wizards of the Coast
Locket of Yesterdays reduces only the amount of mana you need to pay to cast a spell. The spell’s mana cost and mana value are not affected.
No other card is named in this ruling.
#2September 25, 2006· Wizards of the Coast
Colored costs can’t be reduced by this effect. For example, a spell that costs can’t be reduced by more than .
No other card is named in this ruling.
#3September 25, 2006· Wizards of the Coast
The spell being cast doesn’t reduce its own cost, even if you’re casting it from a graveyard. At the time the cost is reduced, the spell is on the stack.
No other card is named in this ruling.