Do you mean “first attempt per day on a single Battle.net account will be eligible to receive the item drop”? Because if its just the first attempt per the entire holiday, if people happen to take the wrong class for a certain epic weapon, that would be quite a missed chance.
I really don’t want to have to look up what a boss drops when a holiday starts, especially for newly-added bosses. I want to play the quest string normally my first time.
My guess would be you have the one chance to get the Satchel per account, and you get increasing chances of getting the mount.
Though it might help to ask for clarification in the GD post:
As CS does not normally give game hints such as this.
Edit, just to add, it starts in just over 90 minutes, guess we’ll find out.
I believe it means that the Satchel drops once per day, on the first character across your whole BNet account. Each subsequent kill will not drop the satchel, but will increase the chances for rare loot in the next day’s satchel.
Example:
Day 1:
Kill Ahune with Character 1 → Satchel drops, no rare items
Kill Ahune with Character 2 → No satchel, but Day 2’s satchel has an increased chance (Lets say 5% for the sake of the example. So, 5% increased chance now)
Kill Ahune with Characters 3-10 → No satchel, but Day 2’s satchel has an increased chance (now 45%, since you’ve killed Ahune 9 additional times).
Day 2:
Kill Ahune with Character 1 → Satchel drops, 45% increased drop chance, get rare item.
Kill Ahune with Character 2 → No satchel, Day 3 satchel gets the 5% increase, resetting because a rare item dropped.
OR
Kill Ahune with Character 1 → Satchel drops, 45% increased drop chance, no rare item.
Kill Ahune with Character 2 → No satchel, Day 3 satchel continues to accumulate another 5%, so now it’s 50%.
So in theory, if you kill Ahune with 21 characters, you can guarantee a drop from the satchel the next day. (Again, 5% is completely made up for the example. No idea what the exact number will be, and they likely will not make that public.)
What Melaesia said 
You get one satchel a day for your Bnet account. However, each alt you kill with after that builds up a “luck bank” that will be spent the next day. So when you do the boss the following day you get a nice increased luck bonus for that satchel.
So, get started building up that magic find and then each day select which char you want to get the Satchel on for your kill.
Blizzard has not said what the % magic find we get per kill is though. At least I don’t think they said.
Well that’s terrible. Some people don’t even have alts.
I personally am an altoholic, but I feel bad for those who aren’t.
Thanks all for the info.
The change would not effect them. It would be the alt hoppers. This just clarifies that you do not have to alt hop on your 20th character to possibly eek out that .00001% chance at a very rare mount.
I don’t think I agree with that. My understanding is that this DOES encourage running alts to build up a Magic Find bonus on your Bnet account. The more alts you run, the more Magic Find you get up to some cap (I assume).
Then the next day, the first char you do the boss with gets to spend that Magic Find % on that Satchel they get.
They have not told us how MUCH of an MF bonus we get per alt char run though so it is hard to tell if it is worth doing on all the alts or not.
Doing it on second character now. It’s giving me the Satchel rewarded listed to queue, guess I was wrong.