When can you use the Druid Circle of the Stars' Cosmic Omen reaction?

Question

I have a question about Circle of the Stars 6th level feature:

Cosmic Omen
When you reach 6th level, you learn to use your star map to divine the will of the cosmos. Whenever you finish a long rest, you can consult your Star Map for omens. When you do so, roll a die. Until you finish your next long rest, you gain access to a special reaction based on whether you rolled an even or an odd number on the die:
Weal (even). Whenever a creature you can see within 30 feet of you is about to make an attack roll, a saving throw, or an ability check, you can use your reaction to roll a d6 and add the number rolled to the total.
Woe (odd). Whenever a creature you can see within 30 feet of you is about to make an attack roll, a saving throw, or an ability check, you can use your reaction to roll a d6 and subtract the number rolled from the total.
You can use this reaction a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

I'm not sure whether I have to use my reaction before I know the score the enemy/party member rolled or I can do it after I know what their score is.

Answer 1

You use your reaction before the roll

Both weal and woe rely on the same trigger (added emphasis mine):

Whenever a creature you can see within 30 feet of you is about to make an attack roll, a saving throw, or an ability check, you can use your reaction to [...]

The feature states you can use your reaction when the creature is about to make the roll, not when that creature is making the roll (which would require clarifications, like what happens with other features like the Bard's Bardic Inspiration) or when the roll has been made.

This reaction happens before the roll (its trigger).