Final answer:
The Amplifying Charm on 25 December, 1997 was cast by Hermione Granger to send a message to Harry Potter in the Harry Potter series.
Step-by-step explanation:
The Amplifying Charm, also known as the Sonorus Spell, was cast by Hermione Granger on 25 December, 1997. This charm is a spell in the magical world of Harry Potter used to magnify a person's voice. In the specific instance mentioned in the question, Hermione used the Amplifying Charm on a coin to send a loud, clear message to Harry Potter when they were at Godric's Hollow. It's an important detail in the series as it showcases the use of practical magic under pressure.