The second sentence is a better one, but it would sound more clear with : 2. The best day to MAKE travel arrangements is WHEN YOU CONFIRM THE DEPARTURE DAY.

Both are correct. Actually, you don't really "book" travel arrangements. You MAKE travel arrangements. You do in fact book travel, though. So if you want to be bookish, #1 is correct. But #2 is also acceptable. BTW, if you get this in a test for advanced English, then only #1 is correct.

what is mean "to book"?????????

the second is better but it needs to be: the best day to MAKE travel arrangements is the day you decide to go also... you book a hotel, a flight and so on and sometimes you hear people saying they booked 'a trip' ('travel' is not a noun but 'trip' is). :)

Though it's longer, I would say that #2 is more understandable, because, in my opinion, #1 feels a bit incomplete reading it. With that said, I think that what blathering said is also accurate. My suggestion would be "The best day to book travel arrangements is the day of departure."