The only celestial encounter I've used in 5E was after the party had just completed a major mission for a deity (they didn't know they were working for him, it was just an adventure for them). While in a church of that god, meeting the bishop who asked them to investigate, an astral deva burst through the stained glass window high above bathed in a bright light, but the shards of glass disappeared before hitting the ground (or the party). The deva had a powerful holy aura that gave a sense of peace to the party, even the evil PC, and when he spoke it was not heard by the ears or the mind, but by the soul. He gave tidings of thanks from his master, and blessed them with a holy gift (+2 Con) that they shall need for the dark times ahead (foreshadowing the next segment of the campaign). The deva then vanished in a flash of light, revealing the stained glass window whole once more.
The players were impressed, and felt the sense of awe I was hoping for... until one of them said "wait... what dark times?!?"