Bungie introduced a new ornament armour sets with Season of the Chosen and many Destiny 2 players will be trying to get them with Bright Dust. This is when you can do so.
Celestial armour sets will be on sale for Bright Dust during five different weeks, on a bi-weekly basis during Season of the Chosen. All three classes can purchase the same part of their armour during the same week.
The first piece that will be sold for BD are the gloves, starting on March 2 , 2021, so we are still more than two weeks out before the first ornament.
Boots will be the next to follow, on March 16, 2021 while the class item ornament will be available in the last week of the month, starting on March 30.
Chest piece ornament will be available on April 6 and you will be able to complete the set with helmets on April 13.
Guardian Games will commence in the week after helmets are sold for Bright Dust so keep that in mind, if you are looking to hoard some of the currency for the themed ornaments. The event will last for three weeks but not all the cosmetics are known at the moment since Bungie classified these files in the API.
If you are looking for more information on what items to expect at which time, you can check the Eververse Weekly page which contains all kinds of useful info to help you plan for Bright Dust earning and spending.