Biblical Calendar: 
The Biblical Calendar is the time reckoning system established by Yahweh (GOD) and was used by the original Hebrew worshippers that later became Israel. The original system reckons time and the calendar that governed when Yeshua (Jesus) went up to Jerusalem to fulfill the Spring Feasts of the LORD. The Biblical Calendar is a lunar calendar based upon the monthly cycles of the Moon’s phases, opposed to the solar calendars, whose annual cycles are based directly on the solar year. Months in the Jewish calendar are based on the phases of the moon. A new month begins on the day of the crescent moon after the New Moon phase.

Gregorian Calendar: 
The Gregorian calendar is the calendar in current use in the Western world, both as the civil and Christian ecclesiastical calendar. Instituted by Pope Gregory XIII in 1582, the calendar has 365 days with an extra day every four years (the leap year.  It is a solar calendar. The oldest solar calendars include the Julian calendar and the Coptic calendar.

Although the Gregorian calendar is in common and legal use in most countries, traditional lunar and lunisolar calendars continue to be used worldwide  to determine religious festivals and national holidays.

The calendar system varies greatly by each different culture. However, if you are looking for detailed information I recommend you visit  the “The Astronomically and Agriculturally Corrected Biblical Hebrew Calendar.” Here is a link https://aroodawakening.tv/biblical-hebrew-calendar/

Calendar Resource: https://aroodawakening.tv/biblical-hebrew-calendar/