Le Japon

Why

Le Japon is masculine and starts with a consonant: à + le contracts into au, which gives au Japon.

Example

Elle part au Japon au mois d'avril.

She is leaving for Japan in April.

Worth knowing

This is the rule in one line: le Japon is masculine and starts with a consonant, so à + le gives au. En Japon is the most frequent mistake.