One of the first things a believer has to do is to accept that the Scriptures as originally Given to Men to write as per the Word of YHWH, were Perfect and the Truth straight from the Mount of YHWH.
Once you come to accept that thought, even if only by faith, you will then be able to start understanding that the ONLY Calendar that matters is the Calendar that YHVH created specifically for His Chosen People.
The Calendar of YHWH first starts being given to us at the very beginning of Genesis and then starts getting more specific with Moses.
Now for those of you that have read the Book of Enoch, you might want to suggest some other thoughts on the matter, but for the time being, we will stick with the current scriptures before looking at what Enoch had to say on the matter.
The first thing to understand is that the Biblical 24 hour day starts with Darkness and not 12 midnight as the Gregorian and other calendears suggest. YHWH made this very clear in the begining of Genesis-Baraysheet.
To keep things simple, the new day always starts at Sunset. There are hours of darkness and hours of light. The lesser light being the Moon, rules the night and the greater light being the Sun, rules the day.
One rotation of the earth on its axis equals One Full Day ( not to be confused with other uses for the term day).One rotation of the Moon around the Earth is equal to One Lunar Month. One rotation of the Earth Around the Sun is equal to One Solar Year. There can be 12 or 13 Solar Months in a Solar Year, but this is not how we build the Calendar of YHWH in regards to our using it to understand the Appointed Times and Feasts of YHWH.
YHWH tells Moses in the Book of Exodus-Shemoth that the biblical year starts on the First New Moon of the month when the Barley in Israel is in its AVIV state of Growth ( Starting to turn golden ), is to be the First Month of the Year. This Command to observe this statute is a Never Ending One and still is in force today.
Now we start to have a problem, because there is great debate on how to determine the Exact New Moon to use and what the New Moon looks like so we can figure the first day of the first month.
Let me say that after much study and prayer on the subject, I can tell you that the New Moon must be totally dark having just come out of being in the light of the Sun. Many have determined that the New Moon should be when you see the first Lighted Cresent which means the moon is coming out of Darkness. This is incorrect and therefore throws the start of the year off by a day or two. (there is proof regarding the Dark Moon in Scripture)
March 17 - Day of Equal (when the daytime equals the nighttime exactly) - The Spring Equinox
March 20 - Day of Equilux (when the Sun crosses the equator) - The Spring Equilux
March 28 - The New Moon of Aviv
April 10 - The Pesah(Passover)- Celebrated at Sunset
April 11 - The Feast of Unleavened Bread
April 16 - First Fruits (the Day Messiah rose from the Dead in that special year)
June 4 - Shavuot(Pentacost)
Sept 21 - Yom Taruah (Day of Trumpets)
Sept 30 - Yom Kippur(Day of Atonement)
Oct 5 - Sukkot(Feast of Tabernackles, Feast of Booths, Feast of the Harvest)
Oct 12 - Shimini Artseret( )
Dec ? - Chanukah(Festival of Lights, Festival of Dedication
March ?? - (March 2018)Purim