Raging Battle Mouse

The second spell you cast each turn costs {1} less to cast.
Celebration — At the beginning of combat on your turn, if two or more nonland permanents entered the battlefield under your control this turn, target creature you control gets +1/+1 until end of turn.

Belligerent of the Ball

Celebration — At the beginning of combat on your turn, if two or more nonland permanents entered the battlefield under your control this turn, target creature you control gets +1/+0 and gains menace until end of turn. (It can't be blocked except by two or more creatures.)

Armory Mice

Celebration — This creature gets +0/+2 as long as two or more nonland permanents entered the battlefield under your control this turn.

Bespoke Battlegarb

Equipped creature gets +2/+0.
Celebration — At the beginning of combat on your turn, if two or more nonland permanents entered the battlefield under your control this turn, attach this Equipment to up to one target creature you control.
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)

Boros Signet

Goddric, Cloaked Reveler

Haste
Celebration — As long as two or more nonland permanents entered the battlefield under your control this turn, Goddric is a Dragon with base power and toughness 4/4, flying, and "{R}: Dragons you control get +1/+0 until end of turn." (It loses all other creature types.)

Pests of Honor

Celebration — At the beginning of combat on your turn, if two or more nonland permanents entered the battlefield under your control this turn, put a +1/+1 counter on this creature.

Tuinvale Guide

Flying
Celebration — This creature gets +1/+0 and has lifelink as long as two or more nonland permanents entered the battlefield under your control this turn.

Anim Pakal, Thousandth Moon

Whenever you attack with one or more non-Gnome creatures, put a +1/+1 counter on Anim Pakal, then create X 1/1 colorless Gnome artifact creature tokens that are tapped and attacking, where X is the number of +1/+1 counters on Anim Pakal.

Gallant Pie-Wielder

First strike
Celebration — This creature has double strike as long as two or more nonland permanents entered the battlefield under your control this turn.

Grand Ball Guest

Celebration — This creature gets +1/+1 and has trample as long as two or more nonland permanents entered the battlefield under your control this turn.

Ash, Party Crasher

Haste
Celebration — Whenever Ash attacks, if two or more nonland permanents entered the battlefield under your control this turn, put a +1/+1 counter on Ash.

Lady of Laughter

Flying
Celebration — At the beginning of your end step, if two or more nonland permanents entered the battlefield under your control this turn, draw a card.