grammar - What is the right way of using ''until''?

Closed until Monday means, it will open at Monday.

But I contract with them until Dec 31st means the contract is valid at 31st...

Why is that?

Also, regarding Closed until Dec 31st:

1. 
   

   Is this option not as clear as using the day of the week?

   
   


2. 
   

   Would it be clearer to say, 'Closing until end of Dec 31st'? (If I want to include that date)

   
   



3. 
   

   Would using the day of the week after make it more clear than using just the date?