The majority of Christians believe that G-d's Holy days were only valid during Old Testament times. Yet, Jesus said in Matthew 5:17-18:
“Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished.
Zechariah 14:16 says we will celebrate G-d's Feast of Tabernacles during the millenium. That's a least one feast G-d has not done away with.
Rosh Hashanah is also called the Feast of Trumpets. I Thessalonians 4:16 says that Christ will return with a trumpet blast. This was the day in which the ancient kings of Israel were crowned. It was determined by the sighting of the new moon which no one knew ahead of time before almanacs and the weather channel (Matthew 24:36). Yom kippur was considered a day of atonement and also one of judgment. It is believed by prophecy experts that this will be the day when Christ judges the nations. Five days later, the Feast of Tabernacles will begin when G-d will tabernacle or dwell among his people. Jesus gave us events to look for when we approached the end. We can kind of guess the season, but will probably never know the year until it actually happens.