The best day that I can't forget is when I made onion cracker. I don't really like onion, but when I made onion cracker, it was so yummy that I couldn't forget it.
 
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Rachel: My best day that I can't forget is when I made onion cracker.
Correct: The best day that I can't forget is when I made onion cracker.
Explanation: You are talking about the best day (superlative form), so we have to use the word the before the superlative adjective best  to show that it is the most unforgettable day in your life.

Rachel:  because I really don't like onion, but when I made onion cracker, it was so yummy that I couldn't forget it.
Correct: I don't really like onion, but when I made onion cracker, it was so yummy that I couldn't forget it.
Explanation: You can omit the word because in this sentence. The word really is an adverb so it should be written before a verb or an adjective. In your sentence, it should be before the word like.

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Thank you for doing your homework Rachel! Keep it up.
Please study the corrections that I have given you.