786. The DIY Yom Tov
If someone experiences a personal nes (miracle) and makes a seudas hoda’ah (a feast of giving thanks to Hashem) that meal is considered a seudas mitzvah (a meal honoring a Divine commandment, similar to a bris or other mitzvah occasion). There are records of community leaders and other people of importance who experienced a personal miracle and would designate that day as a yom mishteh v’simchah (a day of feasting and joy). The Tosfos Yom Tov (Rav Yom Tov Lipman Heller in the 17th century) established Rosh Chodesh Adar II as such a day when he experienced a personal redemption, and poskim say that anyone who experiences a personal salvation should do so as well.
Once someone accepts upon themselves that day as a yom mishteh v’simchah, that acceptance is sufficient for them not to say tachanun (prayers of supplication) in their davening that day.
This would apply even to someone who lived in a community where tachanun was not said on that day, but moved to another community where tachanun is said. They can continue not saying tachanun in their new community on the day that they always celebrated as a yom tov.