How to watch Haruhi

In 2006, Kadokawa/KyoAni didn't know how big the series would be. Thus they prepared 14 episodes and aired them achronologically.

*1 - The Adventures of Mikuru Asahina - Episode 00
*2 - The Melancholy of Haruhi Suzumiya I
*3 - The Melancholy of Haruhi Suzumiya II
*4 - The Boredom of Haruhi Suzumiya
*5 - The Melancholy of Haruhi Suzumiya III
*6 - Lone Island Syndrome (First Part)
*7 - Mysterique Sign
*8 - Lone Island Syndrome (Final Part)
*9 - Someday in the Rain (anime original episode)
10 - The Melancholy of Haruhi Suzumiya IV
11 - The Day of Sagittarius
12 - Live a Live
13 - The Melancholy of Haruhi Suzumiya V
14 - The Melancholy of Haruhi Suzumiya VI

Each episode had a preview for it except the first episode, so when it came time to release the DVDs, Kadokawa released them in 8 volumes mostly chronologically:

1 - The Adventures of Mikuru Asahina - Episode 00
2 - The Melancholy of Haruhi Suzumiya I/II
3 - The Melancholy of Haruhi Suzumiya III/IV
4 - The Melancholy of Haruhi Suzumiya V/VI
5 - The Boredom of Haruhi Suzumiya/ Mysterique Sign
6 - Lone Island Syndrome (First/Final)
7 - Live a Live/The Day of Sagittarius
8 - Someday in the Rain

(Mikuru's Adventure fits in before Live a Live (11th chronologically))

Bandai kept the same episode order on their discs, but also sold a LE that kept the original broadcast order.

When they decided they were to put a significant story as a movie, they realized that they had to air the remaining stories before it, but they couldn't air any that occur after it.

2009 order: (new episodes with * around them)/(no next episode previews)
*1 - The Melancholy of Haruhi Suzumiya I
*2 - The Melancholy of Haruhi Suzumiya II
*3 - The Melancholy of Haruhi Suzumiya III
*4 - The Melancholy of Haruhi Suzumiya IV
*5 - The Melancholy of Haruhi Suzumiya V
*6 - The Melancholy of Haruhi Suzumiya VI
*7 - The Boredom of Haruhi Suzumiya
*8 - *Bamboo Leaf Rhapsody*
*9 - Mysterique Sign
10 - Lone Island Syndrome (First Part)
11 - Lone Island Syndrome (Final Part)
12 - *Endless Eight*
13 - *Endless Eight*
14 - *Endless Eight*
15 - *Endless Eight*
16 - *Endless Eight*
17 - *Endless Eight*
18 - *Endless Eight*
19 - *Endless Eight*
20 - *The Sigh of Haruhi Suzumiya I*
21 - *The Sigh of Haruhi Suzumiya II*
22 - *The Sigh of Haruhi Suzumiya III*
23 - *The Sigh of Haruhi Suzumiya IV*
24 - *The Sigh of Haruhi Suzumiya V*
25 - The Adventures of Mikuru Asahina: Episode 00
26 - Live a Live
27 - The Day of Sagittarius
28 - Someday in the Rain
Movie: ***The Disappearance of Haruhi Suzumiya***

Kadokawa couldn't re-release the 2006 episodes again on DVD, so they released the following episodes on DVD in 2009:
1 - Bamboo Leaf Rhapsody
2 - Endless Eight I/II
3 - Endless Eight III/IV
4 - Endless Eight V/VI
5 - Endless Eight VII/VIII
6 - The Sigh of Haruhi Suzumiya I/II
7 - The Sigh of Haruhi Suzumiya III/IV
8 - The Sigh of Haruhi Suzumiya V

Of course Bandai kept the same episode order on their release. Thus if you wanted to watch the 2009 order on DVDs, you'd have to keep switching discs. Thankfully Kadokawa released the 2009 order on the BD-Box, thus proving that's the proper order to watch it in. Crunchyroll has both seasons (as well as the spinoffs Haruhi-chan and Churuya-san) available for streaming, but they've separated the two as "Season 1" (in broadcast order) and "Season 2" (in 2009 DVD order), thus confusing even more. So this is why we have to ask "Are you going to watch the movie?" and "How are you watching it?" so that we can advise best. Thanks for reading this text dump.