Because ALL SCRIPTURE is still in effect (Mt 5:17), Yahweh's Appointed Times are still to be observed by His people.
Shabbat (seventh day rest) in memorial of Yahweh's rest from Creation, and in obedience to the fourth of the ten mitzvot (commands).
Yom haChodesh (new moon visible crescents) determining the first day of each Hebrew Month (not a day of commanded rest... see 2 Ezra 5:57)
Abib 1 / Yom haShanah — the first day of the Scriptural year in spring, based on the renewal of the moon (lunar cycle) in conjunction with the season of the equinox (solar cycle) and indicated on earth by the ripening of barley - see "Quest For The Calendar Study" also by MattithYah Coover.) Not a sabbath day; work permitted.
Pesach / Chag haMatzah (Feast of Passover/Unleavened Bread) lasting a total of seven days Footnote in memorial of Yahweh's redemption from the death angel and departure from Mitsrayim (Egypt/Bondage), Yahshua's redemption from the curse of our sin (death) and deliverance from shatan's bondage of our souls. (First and last day are high-Sabbaths).
Shavuot (Pentecost), celebrating the giving of the Torah on Sinai, the anointing of the Ruach on the apostles, and the future outpouring of Yahweh's Spirit on all flesh (A High-Sabbath-Day).
Yom Teruah (Day of the sounding of Shofars), in anticipation of Yahshua's imminent return. (High-Sabbath).
Yom Kippur (Day of Atonement / Covering) in thanksgiving for Yahshua's taking our infirmity upon Himself to make atonement for human transgression. (High-Sabbath-Day).
Sukkot / Shemini Atzeret (Feast of Booths and Final Day) for a total of eight days in remembrance of Israel's 40-year wandering in the wilderness, celebration of Yahshua's pitching His tent among us as the seed of Adam, and anticipation of the coming Messianic Kingdom and Wedding Feast of the Lamb (First and eighth days are High-Sabbaths)
Chanukkah (Eight-Day Festival of Dedication or Lights) celebrating the rededication of the temple, Yahshua's rededication of our lives, and his conception/coming as the "light of the world" (Jn 10:22). While an “Optional Feast” established during the Maccabean era and not commanded in Torah, it appears to have been observed by Yahshua and contains specific Messianic significance. (Rest not required, excepting the weekly sabbath which falls therein.)
Purim (Established in the book of Esther and meaning “to cast Lots”, traditionally a celebration of the triumph of good over evil. (A minor-feast, optional, and not a sabbath-day).
American "Holidays" in contrast, are shatan's counterfeit to Yahweh's Appointed Times, being drawn directly from pagan ritual in worship of the father of lies (Jer.10:2). The Roman months and days of the week, likewise, are derived from the names of false deities, which we endeavor not to use in honor of Deuteronomy 12:3,30.