Can you use a Bonus Action on a turn other than your own?

Question

The rules for Bonus Actions (PHB, pg.189) state;

You can take only one bonus action on your turn...

Which to me can be read in two ways.

1. You can only take a bonus action if it is your turn.
2. You can take only one bonus action on your turn but can take one or more on someone else's turn.

As an example, if I were to ready an attack action could I also use a bonus action for Two-Weapon Fighting when the trigger is met?

Answer 1

No

Round and Turn are not synonyms.

From Player's Basic Rules, page 69, or the Player's Handbook, page 189 (bolding added for emphasis):

A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn.

"On your turn" thus specifically excludes other characters' turns.

In the sections on Actions and Bonus Actions, it defines when each is allowed.

On your turn, you can move a distance up to your speed and take one action.

[…]

Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action.

Then, in the section on the ready action (PBR 72, PHB 193)...

To be readied, a spell must have a casting time of 1 action, and holding onto the spell’s magic requires concentration (explained in chapter 10).

A casting time of "1 bonus action" is not the same as "1 Action". In fact, the general casting time is 1 action; a bonus action is specifically called out as different under "Casting Time" on PBR 79 and PHB 202...

Bonus Action

A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn.

So, in looking at the letter of the rules, it's clear that you can't use a bonus action spell outside your turn, and further, it's explicit that your turn ends when someone else begins their turn.

Answer 2

From the Player's Basic Rules, page 69:

Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action.

So bonus actions are meant to be used on your turn. The Ready action talks about preparing an action to be used for your reaction, which should mean 1 action and not allow bonus actions. For further proof, we have on page 72:

To be readied, a spell must have a casting time of 1 action

This explicitly disallows spells with a casting time of one bonus action, which confirms to me that you cannot ready a bonus action.

Answer 3

Well, here's an interesting combination.

The War Caster feat says:

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

The current text of Illusionist's Bracers says:

While wearing the bracers, whenever you cast a cantrip, you can use a bonus action on the same turn to cast that cantrip a second time.

In the Player's Handbook - Combat section, it says this about bonus actions:

Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action. The Cunning Action feature, for example, allows a rogue to take a bonus action. You can take a bonus action only when a special ability, spell, or other feature of the game states that you can do something as a bonus action.

The "on your turn" could be describing what various class features, etc. do, not laying down the law that it has to be on your turn.

The next paragraph says:

You can take only one bonus action on your turn, so you must choose which bonus action to use when you have more than one available.

But again, this is focusing on the limitation of only having one bonus action on your turn.

More importantly, remember from the Player's Handbook Introduction - Specific Beats General rule:

If a specific rule contradicts a general rule, the specific rule wins.

So read Illusionist's Bracers again, and it sure looks like this:

1. On a creature's turn, the creature provokes an opportunity attack from a war caster with Illusionist's Bracers.
2. The war caster casts a cantrip at the creature as a reaction.
3. The Illusionist's Bracers grants a bonus action to the war caster that can be used on that same turn.
4. The war caster casts the cantrip again with that bonus action, not on the war caster's turn.

This should be fixed by saying "on your turn" in Illusionist's Bracers, or the PHB should say explicitly, "A bonus action can only be used on your turn."