After the ED during the preview of the next episode, two episode numbers are announced. Haruhi will say which episode is next, but Kyon will almost always correct her. However, Haruhi is actually not wrong. Kyon states the episodes as they are aired, so the second episode to air is episode 2 (to Kyon that is). The episodes that are shown are not always in order though, and Haruhi states the episode number as if they were shown in order. So if Haruhi says the next episode will be episode 8, but Kyon says it's actually episode 5, the next episode is really the 8th episode that will be aired 5th.

 

So, if the title of an entry is this "Episode 20 (Ep15)", 20 is the order that it is aired and 15 is the real episode number.