I have always understood that all calendars are basically the same, so why does it matter what calendar we keep.

Answer: It is well known among theologians that the Mosaic Hebrews, as well as those living at the time of Yahushua, used the luni-solar calendar for their appointed Feast days. Since Moses recorded both Genesis and Exodus, if there had been a discrepancy between these two, he would have questioned Yahuwah. The Master Yahushua kept all the Feasts and seventh-day Sabbaths according to the Mosaic template.  Another way to be certain that the calendar was the same at Creation, at the Exodus and at the Cross is because the New Year began in spring, which was a symbol of new beginnings.  At the time of Yahushua this same system was still kept.

A new month always began with a new lunation, on New Moon Day. Shocking though it may be, the luni-solar calendar also identifies which day was the true seventh-day Sabbath.

I know this is a new concept to most, and hard to take in, but it's because we have so long followed after the Roman Gregorian calendar, which is far and away different from the Creator's calendar. Consider this: The Seventh-day Sabbath is the first of the Feasts listed in Leviticus 23. The first day of the first week began the day after New Moon Day, while the last day of the week was the Sabbath, which always fell on the 8th, the 15th, the 22nd, and the 29th day of the month. Last, but not least, all the other appointed holy Feasts are located by the luni-solar calendar as well.

The basic difference between the Roman Gregorian calendar and the luni-solar calendar of Scripture is a question of unbroken successive seven-day cycles. The Biblical luni-solar calendar refreshes each month with New Moon Day, followed by four successive week cycles and seventh-day Sabbaths.  The moon also designates specific days as well as when the New Year begins. The Roman Gregorian calendar holds to an unbroken chain of successive weeks, and the sun designates its years. Anyone who is following a calendar that is not in harmony with the Scriptural standard will not be keeping holy the Creator’s true seventh-day Sabbath. Wrong calendar, wrong Sabbath, it’s that simple.  One calendar is man made and the other divine.  For more details, refer to the "Three Months in a Row" video series.