To determine which accessories are available on which days from the accessory merchant in Brilehaven, here is a detailed breakdown:

1. **Metalsday**:
– **Knight’s Bracelet**: Increases Endurance by +6. Costs 5,000 Reeve.

2. **Idlesday**:
– **Rider’s Anklet**: Increases Agility by +9. Costs 9,000 Reeve.

3. **Flamesday**:
– **Stormy Sea Ring**: Always deals critical hits during storms. Costs 15,000 Reeve.

4. **Watersday**:
– **Archetypal Ring**: Increases Archetype EXP gained. Costs 40,000 Reeve. This is the most valuable item for leveling up your Archetypes.

5. **Arboursday**:
– **Rejuvenation Bell**: Provides medium HP recovery every turn in battle. Costs 20,000 Reeve.

The Nidia Shopkeeper sells one accessory per day, and the items available are as follows:
– **Metalsday**: Knight’s Bracelet
– **Idlesday**: Rider’s Anklet
– **Flamesday**: Stormy Sea Ring
– **Watersday**: Archetypal Ring
– **Arboursday**: Rejuvenation Bell

Each day, the shopkeeper offers a different accessory, and you can purchase as many of each item as you want, but the Archetypal Ring is particularly valuable for increasing your Archetype EXP gain.