We use “the big (something)” to describe an event that everyone (or at least the speaker) is very excited about, and has been waiting for. Some examples include:

• the big day
• the big opening
• the big game
• the big show
• the big surprise
• the big reveal.  

Here's an example of how to use "the big ___":

Tomorrow’s the big day!

Using the article “the” shows that both the speaker and listener are familiar with the topic. You could also use a possessive pronoun like “your”:

Your big game is tomorrow, isn't it?