Syrian Orthodox Church of Antioch - Archdiocese of the Western U.S.

Between AD326 and AD1582, Christianity determined Easter using an algorithm approved by the first Ecumenical Council in AD325, with the equinox defined as March 21.

The Julian calendar applies to the original calculation method from 326 AD, which was the first year a unified Easter Dating method was used. The Julian calendar had fallen out of alignment in measuring solar years (keeping months aligned with seasons) well before the Gregorian calendar was introduced. Nevertheless, the Julian calendar is still used today by Orthodox churches as the basis for their Easter.
The Julian Calendar was used by the European (and Christian) communities until the Gregorian calendar became out in 1582.

The Gregorian calendar was first introduced in October 1582 in Italy, and has subsequently replaced the Julian calendar over following years in other European countries.

Since AD 1582 October (when the Gregorian Calendar was adopted by most of Catholic Europe), the Orthodox Easter usually falls on dates different than the Western Christian Easter, although apparently the Churches are discussing using the same formula to determine Easter - probably a formula different than that currently used by either Church.

The Orthodox Easter is determined in the Julian Calendar. It has been claimed that Orthodox Easter does not fall on the date of Passover (15 Nisan in the Hebrew Calendar), or before it; this is true recently, but using the modern formulae for determining the date of Passover (rules which go back to the fourth century A.D.), one finds that, in fact, Easter occurred on the first day of Passover several times before the year A.D. 1000.

The great fast of our Lord precedes the feast of Resurrection commencing on the Monday seven weeks prior. The three day fast of the Ninevites ܨܘܡܐ ܕܢܝܢܘܐ (sawmo d-ninwoyé) begins on the Monday three weeks prior to the commencement of the Great Lent.

Dates of Catholic & Orthodox Easter, ordered by year, in the Gregorian & Julian Calendars from 2005 to 2105

ܡܚܰܘܝܳܢܳܐ ܕ݂ܝܰܘܡܰܝ̈ ܥܐܕ݂ܳܐ ܕ݂ܰܩܝܳܡܬ݁ܳܐ ܕܰܫܢܰܝ̈ܐ ܕܒ݂ܰܝܢܳܬ݂
ܒܗ ـ ܒܩܗ 2005-2105

Table 1.

Years when Western & Orthodox Easter are celebrated the same day, from 2001 to 3000. Notice that after 2698 they are not celebrated the same day.

Years when Orthodox Easter is 1 week after Western Easter, from 2000 to 3000.

2000 2003 2006 2009 2012 2015 2018 2019 2020 2022 2023 2026 2029

2030 2033 2036 2039 2042 2044 2047 2049 2050 2053 2056 2057 2060

2063 2064 2066 2067 2068 2074 2076 2077 2080 2083 2086 2087 2088

2090 2093 2094 2098 2101 2104 2106 2107 2110 2113 2114 2117 2118

2120 2121 2123 2124 2125 2128 2131 2133 2134 2137 2140 2142 2143

2144 2145 2147 2148 2150 2151 2152 2155 2158 2161 2162 2164 2167

2169 2170 2171 2172 2174 2175 2178 2181 2182 2185 2188 2189 2191

2193 2194 2196 2199 2201 2202 2205 2208 2212 2213 2215 2216 2218

2219 2220 2223 2226 2229 2232 2235 2237 2238 2239 2240 2242 2243

2245 2246 2250 2253 2256 2257 2259 2262 2264 2265 2267 2269 2270

2273 2276 2277 2280 2283 2284 2286 2288 2289 2291 2294 2296 2297

2299 2300 2303 2307 2308 2310 2311 2313 2314 2318 2319 2321 2324

2327 2330 2332 2333 2335 2337 2338 2340 2341 2343 2345 2348 2351

2352 2354 2357 2360 2362 2364 2365 2367 2368 2371 2375 2378 2381

2384 2386 2387 2389 2392 2394 2395 2398 2402 2405 2406 2408 2409

2410 2411 2413 2414 2416 2417 2419 2421 2422 2425 2428 2429 2430

2432 2433 2435 2436 2438 2440 2441 2443 2446 2448 2449 2451 2452

2455 2457 2459 2460 2462 2463 2465 2466 2467 2470 2471 2473 2476

2479 2481 2482 2484 2485 2486 2487 2489 2490 2492 2493 2495 2497

2500 2501 2503 2504 2506 2508 2509 2511 2512 2514 2516 2517 2519

2520 2523 2525 2527 2528 2530 2531 2533 2535 2536 2538 2539 2541

2544 2546 2547 2550 2552 2554 2555 2557 2558 2560 2561 2563 2565

2566 2568 2571 2574 2576 2577 2579 2580 2582 2584 2585 2587 2588

2590 2592 2595 2596 2598 2599 2601 2604 2606 2607 2609 2611 2612

2614 2615 2618 2620 2622 2625 2626 2628 2630 2631 2633 2634 2636

2639 2641 2642 2645 2649 2650 2652 2653 2655 2656 2658 2660 2661

2663 2666 2668 2669 2672 2674 2675 2677 2679 2680 2682 2683 2685

2687 2690 2693 2694 2696 2699 2701 2702 2704 2706 2707 2709 2710

2712 2713 2715 2717 2718 2720 2721 2723 2726 2728 2729 2731 2732

2734 2736 2737 2739 2740 2742 2744 2745 2747 2748 2750 2751 2753

2755 2756 2758 2759 2761 2763 2764 2766 2767 2769 2770 2772 2774

2775 2777 2778 2780 2782 2783 2785 2786 2788 2789 2791 2793 2794

2796 2797 2799 2801 2802 2804 2805 2807 2808 2810 2812 2813 2815

2816 2818 2821 2823 2824 2826 2827 2829 2831 2832 2834 2835 2837

2839 2840 2842 2843 2845 2846 2848 2850 2851 2853 2854 2856 2858

2859 2861 2862 2864 2865 2867 2869 2870 2872 2873 2875 2877 2878

2880 2881 2883 2884 2886 2888 2889 2891 2892 2894 2896 2897 2899

2900 2902 2903 2905 2907 2908 2910 2911 2913 2916 2918 2919 2921

2922 2924 2926 2927 2929 2930 2932 2935 2937 2938 2940 2941 2943

2945 2946 2948 2949 2951 2954 2956 2957 2959 2960 2962 2964 2965

2967 2968 2970 2973 2975 2976 2978 2979 2981 2983 2984 2986 2987

2989 2992 2994 2995 2997 2998 3000

Notes on the Orthodox Easter

Table 1.

Table 2.