Yes, that phrase, Learn Atlassian day, is idiomatic.
The word day can be modified by a clause. Compare:
Bring your Child to Work Day
National Cut Your Energy Costs Day
National Girl Hug Boy Day
Kiss a Ginger Day
National Clean Your Desk Day
National Wear Red Day
Shower With A Friend Day
Read Across America Day
National Be Heard Day
National Let's Laugh Day
The clause is understood to function like an adjective. The day is a day on which the action of that clause is meant to be deliberately engaged in.
The day can happen once a year, monthly, or weekly, or at random or arbitrary times.
Monday is our take out the garbage day.
The fifteenth is our monthly clean out the office fridge day.
We're going to have another clean up the playground day in the near future.
The pattern can be paraphrased like this:
Monday is our day to take out the garbage.
The fifteenth is our monthly day to clean out the office fridge.
We going to have another day in the near future to clean up the playground.