A “feast” is usually a private party held by a guild at its headquarters or more often a rented venue, for members (and their guests, generally a single guest of the opposite gender) only - - OR it’s a private party held by a noble family for a select guest list, usually in their own city mansion or villa. Feasts are usually dominated by a sit-down meal, much drinking, and chat, and has small-scale hired entertainments (minstrels, jugglers, hedge-wizards). Many are intended to end in dalliance in various gardens or rooms.
A “revel” is a much larger social event held by a noble family, either in their own villa or at a large and luxurious rented location. It has a far more open guest list than a feast, or may even be an “open to all” event (if the latter, the Watch and Watchful Order will send out-of-uniform peace-keepers to bolster the hired security the hosts inevitably lay on). Revels may have dancing with music, and may have hired orators or actors or tumblers (acrobats), but are usually dominated by standing and chatting while servants (of the hosts’ household, augmented by many hired servants; you’ll see a glimpse of this in the forthcoming novel Elaine and I wrote) make the rounds with endless platters of food and drink. Guests are always “announced” by senior servants as they enter the venue (through a doorguard, to keep out gangs of street youths or thugs bent on thievery or pranks).
A few revels and feasts have costumes, or masks - - usually “doffed at deepnight” (taken off at midnight) - - and some of these even have ‘themes’ (costumes must all be piratical, or as a Tashlutan harem is imagined by Waterdhavians to be). Waterdeep has a long and colorful history of debauched celebrations, with wealthy families occasionally trying to outdo each other, and they serve as entertainments for the entire city (get injured or very drunk at a revel, and show up late for work the next day, and your employer may well be amused or even proud at your state - - provided you tell him and any guests all about what happened at the revel, over and over, until every questioner’s curiosity has been satisfied).