Today is the 2nd day of Rosh HaShanah. It was only to be a one-day feast, celebrated on the day of the new moon, on the first day of the seventh month of the Jewish calendar. All the other feasts God commanded His people to observe either start on the night of the full moon or are counted from other fixed feast days. But Rosh HaShanah begins on the night of the new moon.
The new moon is the first visible crescent moon after the moon has been completely hidden from earth because of its direct (or very nearly direct) alignment between the sun and the earth. The sun is then shining completely on the back side of the moon, so there is a period of time when we can't see the moon at all.
The exact timing of the new moon is hard to nail down. (See the formulas below if you doubt that!) Due to this difficulty, Rosh HaShanah became a two-day feast. Because it is based on the cycles of the moon rather than on a calendar date, the feast day is movable; that is, it can fall anywhere from September 5 to October 5.
In the Bible Rosh HaShanah is called the Feast of Trumpets. It is the first of the fall feasts. There are three spring feasts to start the new year, then a fifty-day space before the middle or fourth feast, and three fall feasts. There's a beautiful symmetry to the seven feasts and much could be said about their precision and alignment, but that will have to be saved for another time. Suffice it to say, the feasts are completely interwoven with astronomy, they are called the Feasts of the Lord (not as many erroneously call them, the Jewish Feasts—though they were given to the Jewish people under Moses' leadership; and because of that, it is primarily the Jewish people who celebrate them), and the first four feasts, we know from the New Testament, had prophetic meaning and symbolism.
The prophetic meanings of all four spring-summer feasts were fulfilled on their exact days by Jesus Christ: His death, His resurrection, and the sending of the Holy Spirit. That being the case, does it not seem logical that the prophetic meanings of the fall feasts would also find their fulfillment, on the exact days, in the second coming of Christ? After all, they are the Feasts of the Lord, designed by the Creator of the universe with all its astronomical and historical movements.
Most Christians will object to pointing to a calculated day because of what Jesus told His disciples: Of that exact day and hour no one knows, not even the angels in heaven, but the Father only (Matthew 24:36). But consider this possibility: given that the Feast of Trumpets is the only feast of the seven in which the exact day and hour is unknown, perhaps Jesus was making a pointed reference to the Feast of Trumpets.
There isn't much said in the Bible about the Feast of Trumpets (Leviticus 23:23-25 and Numbers 29:1-6), but there are many traditions surrounding this feast day. Here are a few:
It is first of all a day for the blowing of trumpets (Numbers 29:1). The sound of the trumpet is a wake-up call, warning us, getting our attention, helping us think about God, awakening our conscience. If you have a shofar or a trumpet, blow it as a call to watchfulness and to repentance.
It is commemorated as the final day of the six days of creation, the crowning and concluding day of all God made. So we can use the day to rejoice in all God has made, and worship Him for the wonders of creation.
It is also the day to remember that God is King. (Psalms 92-100 are good reading for that.)
It is the day to remember the sacrifice of Isaac and God's redemptive salvation in providing the Lamb of God. Traditionally, Genesis 22:1-19 is read.
It is celebrated as the first day of the civil New Year (whereas the month of Passover is considered the first day of the calendar year -- similar to our New Year being in January, but the start of our yearly planning, life routines, etc., is in September).
It is a day to celebrate the goodness and sweetness of life. Traditional foods include sliced apples dipped in honey, dates, challah bread with raisins, honey cake. Traditional blessings and greetings state: "May you have a sweet New Year."
It is also the first day of the Ten Days of Awe, the 10-day period between the Feast of Trumpets and the Day of Atonement, known as Judgment Day. So, while Rosh HaShanah is a joyful, festive day, it is also a solemn day. Use these next few days to ask yourself some hard questions. What does repentance look like? What does the Bible say are the elements of true repentance? How can one’s name be written in the Book of Life? What will we be judged for? Who is the Judge? Am I pleasing God by my choices each day? Am I living the way God wants me to? Is there anyone I have offended/wronged from whom I need to seek forgiveness? Have I fulfilled my word? Is there anything for which I need to make restitution? Is there anyone with whom I am not reconciled?
For me, it's a day to remember that Jesus has promised to return (John 14:1-3). I Thessalonians 4:16 tells us that Christ will return with the sound of the trumpet. It's a day to reflect on my own readiness for His second advent. Last night I read Luke 12:31-48 to my two daughters who are still at home. And I had to ask myself this from verse 36: When He comes, will I be ready to open the door immediately and welcome Him? Do I have a watchful, expectant, eager, prepared—no last minute flurry to tidy things up, to throw a few things in the closet so He won't see—open, transparent heart, ready to joyously fling open the door for the Savior I long to see?
So, eat some apples today dipped in honey, read one of the "coronation" Psalms (92-100), read Genesis 22, and remember Christ's promise to come again. But mostly, take some time to reflect, to ask God to search your heart, to make you ready to see Him face to face.
Wikipedia says this about New Moon calculation:
The length of a lunation is about 29.53 days. Its precise duration is linked to many phenomena in nature, such as the variation between spring and neap tides (the most and least profound tidal variances respectively). An approximate formula to compute the mean moments of new moon (conjunction between Sun and Moon) for successive months is:
where N is an integer, starting with 0 for the first new moon in the year 2000, and that is incremented by 1 for each successive synodic month; and the result d is the number of days (and fractions) since 2000-01-01 00:00:00 reckoned in the time scale known as Terrestrial Time (TT) used in ephemerides.
To obtain this moment expressed in Universal Time (UT, world clock time), add the result of following approximate correction to the result obtained above:
Periodic perturbations change the time of true conjunction from these mean values. For all new moons between 1601 and 2401, the maximum difference is 0.592 days = 14h13m in either direction. The duration of a lunation (i.e. the time from new moon to the next new moon) varies in this period between 29.272 and 29.833 days, i.e. −0.259d = 6h12m shorter, or +0.302d = 7h15m longer than average. This range is smaller than the difference between mean and true conjunction, because during one lunation the periodic terms cannot all change to their maximum opposite value.
The long-term error of the formula is approximately: 1 cy2 seconds in TT, and 11 cy2 seconds in UT (cy is centuries since 2000.