I always saved it until after actually completing ME3. Citadel was a swan song for the trilogy, giving you one last, decidedly less dreary adventure. There is a moment at the very end, which marks it as the perfect ending for the series. Won’t mention what, in case you’ve never played it.

Definitely before the end. I personally hate the little ‘one mote story’ scene. It only works for Refuse ending

After the Geth Dreadnought or even Rannoch seems prudent. Shep's been through a lot at this point and anything after Thessia feels to urgent to ... take a break.

As you have access to some of the best mods and 2 superb weapons by playing the story.... I wouldn't wait to long.

This however does not apply to the party. I always do it after horizon, as this way, everyone is available.... if they are alive that is.

Right before you start the final mission: Priority: Cerberus Headquarters.

And I suggest you to complete all the sub tasks (otherwise you’ll can’t be able to invite most of the ME2 squadmates )

If you inherited the savings through ME1 & ME2. Only Kaiden / Ashley (depends who you decide to save on ME1), Mordin (died for saving the Krogan species on ME3, but you’ll receive his songs after the party night) ,Thane (hurt badly in the fight with Kai Leng, then died in hospital) , Legion (died to give all geth intelligence on Rannoch in ME3) can’t be invited. And you’ll get a family photo before the DLC end.

Oh , by the way. If your romance relationship is with Tali (from ME2 to ME3) , you’ll heard the most beautiful song in this game.

This DLC should be completed before you head to the Cerberus HQ.

Before heading to the Cerberus base. It's best since if everyone is alive they will all be able to show up in some way, and it's the funniest place to do it. Since it's like at the very ending of this long desperate war and Shepard and crew are basically like "Screw time for a break." No matter what, it makes no sense to play it at any point since it's basically a joke so just play it before you enter the Cerberus Base.

If you're going to put it inside the rest of the game, then do it after Horizon, that way you can have everybody there (Miranda is only available after Horizon).