Friday Comes Before Thursday

Guess what comes next in the sequence:

1 2 3 4 5 ? 

The answer is not 6. What is the answer?

What goes up but never comes down?

What am I?

Which 3-letter word can be inserted into all four lines above to form complete words?

P_ _ _ N T

D E _ _ _ S T

S T _ _ _

C _ _ _ E R

What goes up and never comes down?