How to use this table:

The year:

The Gregorian lunar almanac (also called the Gregorian "ecclesiastical moon") is a 19-year cycle which assigns an age of the moon to every day in the 19-year period.

Year one of the Gregorian cycle is any year in which the year's number is divisible by 19 without remainder.  The year 1995, for example, was year one of its cycle:  The table shows that Lunar Year 1995 began on January 2nd, 1995.  (More precisely, at sunset on January 1st, 1995).   Successive years of the cycle follow in order.  The year 1996 was year two:  Lunar Year 1996 began on December 22nd, 1995  (Or more precisely, at sunset on December 21st, 1995).   The year 1997 was year 3:  Lunar Year 1997 began on December 11th, 1996.
 

The month:

Each year contains 12 or 13 lunar months.  Each lunar month has formally 29 or 30 days.   The table above shows the first day of each lunar month in the Gregorian Easter cycle.  The moon is considered to be one day old on the first day of the lunar month.  This day is also called "new moon" because it corresponds, on the average, to the day on which the new waxing  crescent moon theoretically first becomes visible.

Although each lunar month contains formally either 29 or 30 days, in the Gregorian lunar almanac the actual number of days assigned to a given lunation might be as few as 28  (when the almanac assigns lunar ages differing by 2 to successive days in a month formally having 29 days) and as many as 31 (when the almanac assigns the same lunar age to successive days in a month formally having 30 days).  If one simply uses the starting dates listed in the table above, these complications will be taken care of automatically. However, if one wishes to use the Gregorian lunar almanac not just as an almanac, but as a calendar, in which every day is designated by a unique and unambiguous date, additional rules may be needed, such as the rules enumerated below for February 29th, for the 13th moon of the 19th year of the cycle, and for the 12th moon of the year 2199.
 
 
 

Transition from Almanac to Calendar--Additional Rules:

These additional rules are used to adjust the Gregorian lunar almanac to conform to the following constraints:  (1)  Every lunar month will have exactly 29 or 30 days numbered consecutively; (2) each day is uniquely identified by the number of the year, the  number of the month, and the number of the day; no day does double duty as the last day of the outgoing month and the first day of the incoming month;  (3) the number of variable months, which can have either 29 or 30 days, is to be no more than 4.  The approach taken here is to designate months 2, 11, 12, and 13 as variable.  This allows any year of 12 months to be reduced in length by a day, and any year of 13 months to be reduced in length by as much as two days, and any year, whether of 12 or 13 lunar months, to be increased in length by as much as two days,.  A more flexible approach would designate months 2, 9, 11, and 12 as variable, allowing any year whatsoever to be reduced or extended by as much as two days. It will be centuries before this additional flexibility is needed,  however, so the table above follows the traditional approach of making the 13th month variable.

The additional rules themselves are subject to the constraint that the Gregorian lunar calendar (almanac plus additional rules) must set the Easter holiday to the same date as the unadjusted Gregorian lunar almanac.

Additional Rule 1:  If February has 29 days, an extra day is added to lunar month 2, making it a month of 30 days, rather than 29 days.  Month 3 then begins a day later than the date listed in the table if the date listed is a date prior to February 29th.  (The unadjusted almanac compensates for February 29th by declaring the moon's age on February 29th to be the same as her age February 28th; or, in the alternative, by declaring the moon's age on February 25th to be the same as her age on February 24th in a bissextile year.)

Additional Rule 2:  The thirteenth lunar month of the 19th year of the cycle has 29 days in the years 1900-2199.  (Though this saltus lunae or moon's jump is already part of the unadjusted almanac, this rule is added to ensure that the days will be numbered consecutively.)

Additional Rule 3: In the year 2199, which is the 15th year of the 19-year cycle and the last year in which this table is valid, an extra day is added to lunar month 12, making it a month of 30 days, rather than 29 days.  (The unadjusted lunar almanac manages the transition from the year 2199 to the year 2200 by setting the moon's age on January 1st, 2200 to be the same as her age on December 31st, 2199.)
 

The day:

Because this is a Babylonian-style lunar calendar, the day begins on sunset of the day prior to the day listed in the table.  The table shows that the first day of the first moon of year 1 of the cycle corresponds to January 2nd.  This means that the first day  of the lunar month begins at sunset on January 1st and ends at sunset on January 2nd, when the second day of the lunar month begins.
 

The Easter festival falls in the first lunar month of the year to begin on or after March 8th.  This is usually the 4th moon, though in years 6 and 17  of the 19-year cycle it is the 5th moon.  Easter is always the third Sunday in its lunar month.

In the Earth's Northern hemisphere, the Spring season of year 4 of the Gregorian cycle coincides with the Spring season of year 1 of the Jewish 19-year cycle, with other years of each cycle following in succession.  The Feast of Unleavened Bread for the year 5754 (year 16 of the Jewish cycle) came in the Northern hemisphere's springtime in the Gregorian year 1994 (year 19 of the Gregorian cycle.)

Due to differences in the time of year at which the Christian and Jewish cycles add the 13th moon of 13-moon years, the Jewish Feast of Unleavened Bread will, in years 3, 11, and 14 of the Gregorian cycle (corresponding respectively to years 19, 8, and 11 of the Jewish cycle), fall in the lunar month next after the lunar month in which the Christian Easter festival falls.  In all other years of the cycle, Easter and the Feast of Unleavened Bread fall in the same lunar month.  But though the lunar month in which the Jewish Feast of Unleavened Bread falls can be identified using the table above, the precise dates of the Feast of Unleavened Bread cannot be determined accurately from the table.  This is because the Gregorian and Jewish calendars use different rules for computing the moon's age.

References:

Explanatory Supplement to the Ephemeris and the American Ephemeris and Nautical Almanac, H. M. Stationery Office, London, 1966, fourth revised printing, 1977.

Alexander Philip, The Calendar: Its History, Structure and Improvement, University Press, Cambridge, 1921.

L. A. Resnikoff, "Jewish Calendar Calculations I", Scripta Mathematica 9, 191(1943); "Jewish Calendar Calculations II", Scripta Mathematica 9, 274(1943).