Phineas and Ferb Christmas Vacation!

Danville was marked naughty because Dr. Doofenshmirtz activated the Naughty-inator, which sent the entire city's name to Santa's Naughty list. The two elves who investigated later clarified that it was the city itself, not its residents, that was classified as naughty, which is why the people living there were still affected by the cancellation of Christmas gifts.

Earlier in the episode, Doofenshmirtz receives the Naughty-inator as a gift from his uncle, establishing the device's existence before its activation.

The Naughty-inator was destroyed when Doofenshmirtz played a Christmas CD by Sal Tuscany. The vibrations from the song 'Christmas cannot be destroyed not even by a Naughtyinator!' caused the device to malfunction and explode, lifting the naughty label from everyone in Danville and restoring Christmas cheer.

Perry received the Sal Tuscany CD as his Secret Santa gift at the OWCA Christmas party, making its later appearance in Doofenshmirtz's lair a key plot point.

The Sal Tuscany CD was actually a gift from Santa Claus himself. The name 'Sal Tuscany' is an anagram of 'Santa Claus', the 'Y' in the original was meant to hide this identity. When Doofenshmirtz discovered this, he realized Santa may have intended the whole incident to help him find holiday happiness.

The CD was given to Perry as a Secret Santa gift at the OWCA party, but its true origin remained hidden until the reveal.

Doofenshmirtz initially struggled with whether to activate the Naughty-inator because he admits he doesn't actually dislike Christmas. He ultimately caved to pressure from carolers who came to his door demanding figgy pudding and singing 'We Wish You a Merry Christmas,' prompting him to activate the device just to make them leave.

Doofenshmirtz's reluctance to use the device is shown when he tells Perry he's still deciding whether to activate it, highlighting his internal conflict.

Buford, who had been on the Naughty list, managed to get the entire town to turn on their Christmas lights simultaneously. This act of spreading holiday cheer automatically transferred him to the Nice list, allowing him to receive presents like everyone else.

Earlier, Buford was established as someone on the Naughty list, setting up his motivation to perform this redemption act.

Santa visited Danville because Phineas and Ferb had built an elaborate rest stop on top of their house specifically for him. Despite the Naughty-inator's effects being reversed, Santa had already completed his gift deliveries and decided to stop at the boys' creation to thank them for their help before flying off with the rest stop hitched to his sleigh.

The entire episode establishes Phineas and Ferb's elaborate Santa rest stop as their holiday project, which included a sauna, massage table, satellite TV, and reindeer feeding station.

Candace sold the necklace she had been planning to turn into earrings for Jeremy in order to buy him a new guitar as a Christmas present. In return, Jeremy sold his old guitar to buy Candace the earrings she wanted, symbolizing their mutual love and the true spirit of giving.

Candace was established as searching for the perfect gift for Jeremy throughout the episode, and her quest for the ideal present was highlighted in the song 'What Does He Want.'

Baljeet got his Christmas wish when he was accidentally launched from the flying sleigh's jammed present cannon, sending him flying down a chimney. He landed under the mistletoe where he received a tender kiss from a girl, fulfilling his holiday wish.

The kids struggled with visibility in the dark and a jammed present cannon on their flying sleigh, which led to this accidental but fortunate outcome for Baljeet.

Perry the Platypus gave Doofenshmirtz a piece of almond brittle as his Christmas gift, symbolizing the show's tradition of unexpected friendship between the two. Even though they are enemies, Perry's gesture represents the holiday spirit of kindness and forgiveness, and almond brittle is Doofenshmirtz's favorite treat.

Perry's tradition of giving gifts to his nemesis despite their adversarial relationship has been established in previous episodes of the series.

After the Naughty-inator was destroyed but it was deemed too late to fix the mistake with Santa, Phineas, Ferb, and their friends built a flying sleigh with help from the elves to deliver presents to every household in Danville themselves. They successfully distributed gifts across town despite challenges with darkness and technical difficulties.

The elves told the kids it was too late to undo the damage with Santa, prompting them to take matters into their own hands.