Does the duration of the invisibility from the Firbolg's Hidden Step trait allow me to make an attack roll with advantage on the following turn?

Question

The firbolg's Hidden Step trait (VGtM, p. 107) is described as being active "until the start of your next turn" at most:

As a bonus action, you can magically turn invisible until the start of your next turn

I'm unclear on the exact mechanics of turn starts. For instance, if I use Hidden Step as a bonus action at the end of my turn, would the invisibility stay active for my next attack?

My initial reading was "no" as the trait would stay active from when I use it, into the next round, and drop as soon as my turn came up in the initiative order. I would then become visible, and take my action. This also gels conceptually: I can use Hidden Step to protect myself from combat for a round, or I can use it to gain advantage on an attack. But I can't use it for both.

However, I've also read that advantage for invisibility is determined at the start of the round, and thus the advantage would in fact carry over to the next round's attack, as if I was attacking from hiding or something similar.

Which is it, and what source would resolve it?

Answer 1

Your first idea is correct: Hidden Step ends when your turn starts and invisibility does not grant advantage throughout a round but is instead checked with each attack you make

This is clear from the rules on "Unseen Attackers and Targets" which states:

[...] When a creature can't see you, you have advantage on attack rolls against it. [...]

And then the section on "Making an Attack" states:

Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure. [...]

[...] 2. Determine modifiers. The DM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll. [...]

This shows us that (dis)advantage is determined on a case-by-case bonus as the attack is made; meaning that the advantage granted by invisibility does not persist throughout a round but instead it stops granting advantage the moment you become visible, which, in this case, is at the start of your turn.