KanjiJLPT N5
Hit

hit, right, appropriate, himself

Mnemonic

You hold a burning candle over a comb, letting wax harden between its teeth. Flick each tooth and every glob launched is a direct hit, striking the target across the room dead center!
Additional thoughts
Picture the satisfying thwack of wax pellets hitting a bullseye. The comb becomes a tiny catapult powered by candlelight—every shot lands perfectly on the mark.
Quick recall
Flick wax from a burning candle off a comb—every shot's a hit!

Details

The keyword for 当 is hit. This kanji conveys the idea of striking a target or being on the mark, encompassing meanings of accuracy, appropriateness, and relevance. It extends to the sense of something being applicable or corresponding to a situation, as in being the right fit or being assigned to a task. It also carries the nuance of winning, as in hitting the mark in a lottery or guess.
On'yomi
とう
Kun'yomi
あ.たる、あ.たり、あ.てる、あ.て、まさ.に、まさ.にべし