"The day is here." - What does it mean?
What does this sentence mean exactly?

Does it mean you have reached to the d-day of certain event you've been waiting for?
If yes, can I say like "I will not leave my house til the Christmas day is here."?

Or does it mean sth else? How do you use this sentence?

Question

"The day is here." - What does it mean?
What does this sentence mean exactly?

Does it mean you have reached to the d-day of certain event you've been waiting for?
If yes, can I say like "I will not leave my house til the Christmas day is here."?

Or does it mean sth else? How do you use this sentence?

Answer

Yes, it means that you have reached that day.
In your example - 
"I will not leave my house until Christmas Day is here." (no "the")  It means you will stay inside until December 25.

Answer

Great! Thanks!

Answer

The time has arrived. E.g. I was waiting for my opportunity, and now the day is here