What's "today is the day" mean? In what contexts can it be used?

Native speaker’s answer
Rebecca
"Today is the day" is an expression one can use to indicate that they are confident about successfully completing a task/challenge or that the conditions are appropriate for something. They may have attempted the task or challenge in the past, but this is not necessary. Ex: Today is the day I conquer my fear of heights. I'm going rock climbing. => confidence Ex: Today is the day we can finally go to the beach. It's not raining for once. => conditions