The spell Holy Weapon empower a "weapon you touch" to do extra damage (2d8 radaint per hit) and shed light (30 feet bright light, 30 feet dim light). It has one final feature (XGtE, p. 157, bold added):

As a bonus action on your turn, you can dismiss this spell and cause the weapon to emit a burst of radiance. Each creature of your choice that you can see within 30 feet of you must make a Constitution saving throw. On a failed save, a creature takes 4d8 radiant damage, and it is blinded for 1 minute.

Now, note that unlike some spells (like Shillelagh), this spell doesn't end if you are no longer touching the weapon you cast it upon. So you could conceivably cast this spell on an ally's weapon (say, if you're a cleric and you have a fighter ally who attacks many times in one turn). If you did this, I found myself wondering where the "burst of radiance" would be centered: on the spellcaster, or on the weapon?

In seemed unambiguous at first, since every time a spell mentions "you", it's referring to the caster. But at the same time, the spell says that the burst of radiance comes from the weapon, so I became unsure.

So how would you rule this? If the person who casts Holy Weapon is far away from the weapon when they activate the "burst of radiance" where is the burst centered? On the spellcaster, or on the weapon?

The burst is intended to originate from the weapon.

Xanathar's Guide to Everything's lead designer and managing editor Jeremy Crawford noted on Twitter:

The explosion of the holy weapon spell originates from the weapon, not
the spellcaster. The text that says otherwise will be corrected in a
future printing. #DnD

The text may or may not yet be corrected in the entry for holy weapon on D&D Beyond or current printings of the book, but either way, that will require an additional purchase.

Currently, there is no errata available online for Xanathar's Guide to Everything.

• 
  November 2017 Errata (Princes of the Apocalypse, Elemental Evil Player's Companion)


• 
  November 2018 Errata (Player's Handbook, Monster Manual, Dungeon Master's Guide)

Despite preferring RAW, I would rule it originates from the weapon.

Though I tend to rule fairly strictly to the rules text, this spell's description has a clear mistake that also violates the fiction and flavor of the spell. I have played with both interpretations (before noticing the error and after), and I don't think changing the burst's origin significantly changes the power of the spell.

What if I want me to burst?

If I had a player who was strongly attached to the literal text interpretation, I would allow it, so long as the burst's origin is consistent. I would not allow the added flexibility of choosing the burst's origin spontaneously.

Oddly enough, it's you

As you've quoted, the specifics are within 30' of you and not within 30' of the weapon. Even though it talks about weapon emitting the radiant damage, the spell is clear that it only affects creatures within 30' of you (the caster.)

Ruling otherwise

I think a roleplay case could be made for using the weapon as the point, but that may give 'more range' to the damage than originally assumed and removes your own risk of being close to whatever it is you want to damage. This may be minimal, but you could easily 'game' this by having something like an unseen servant or some other creature deliver the weapon to it's explosion point and turn this into a remotely detonated bomb. Having said that, I don't think it'll be gamebreaking to have it emit from the weapon, but if you're looking at the RAW, then the caster is the centerpoint.

The RAI (and revised RAW) is that the Weapon is the origin of the Damage + Blindness, not the Spellcaster

Jeremy Crawford has confirmed that the text in Holy Weapon is a mistake, and would be corrected in future printings:

The explosion of the holy weapon spell originates from the weapon, not the spellcaster. The text that says otherwise will be corrected in a future printing. #DnD

—Jeremy Crawford, Twitter, 2018-03-14@6:37PM EDT

I don't know if the revised version has already been printed, or if Errata has been released, or what the revised wording will look like. I predict, however, it'll probably be something like this:

You imbue a weapon you touch with holy power. Until the spell ends, the weapon emits bright light in a 30-foot radius and dim light for an additional 30 feet. In addition, weapon attacks made with it deal an extra 2d8 radiant damage on a hit. If the weapon isn't already a magic weapon, it becomes one for the duration.

As a bonus action on your turn, you can dismiss this spell and cause the weapon to emit a burst of radiance. Each creature of your choice that you can see within 30 feet of you the weapon must make a Constitution saving throw. On a failed save, a creature takes 4d8 radiant damage, and it is blinded for 1 minute. On a successful save, a creature takes half as much damage and isn't blinded. At the end of each of its turns, a blinded creature can make a Constitution saving throw, ending the effect on itself on a success.

—Possible Holy Weapon Revision