Important
Valuable
noun, な adjective
You know 事 means "occurrence," but it can also mean "matter." And if something is a big matter to someone, it must be a very important and valuable thing to them.
大事 describes something that holds great importance or value. For example, you can use 大事 to describe something or someone that matters a lot to you. You can also use 大事 to describe a crucial task or moment. And if you want to bring up a serious topic, you may say that you have something 大事 to talk about.
This is a jukugo word, which usually means on'yomi readings from the kanji. Since 大 has two on'yomi readings, here's a mnemonic to help you remember which one to use:
Nothing in your life is as important or valuable to you as dyes (だい). You really just love dyes more than anything else. Dyes, dyes, dyes!
きちんと「ありがとう」と言うことは大事です。
It is important to say "thank you" properly.
大事なノートを失ってしまった。
I lost an important notebook.
明日は大事な大会なので、早くベッドに行きます。
Tomorrow is an important tournament, so I will go to bed soon.