Year 321 (CCCXXI) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. In the Roman Empire, it was known as the Year of the Consulship of Crispus and Constantinus (or, less frequently, year 1074 Ab urbe condita). The denomination 321 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
| Millennium: | 1st millennium |
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| Centuries: |
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| Decades: |
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| Years: |
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| Gregorian calendar | 321 CCCXXI |
| Ab urbe condita | 1074 |
| Assyrian calendar | 5071 |
| Balinese saka calendar | 242–243 |
| Bengali calendar | −272 |
| Berber calendar | 1271 |
| Buddhist calendar | 865 |
| Burmese calendar | −317 |
| Byzantine calendar | 5829–5830 |
| Chinese calendar | 庚辰年 (Metal Dragon) 3018 or 2811 — to — 辛巳年 (Metal Snake) 3019 or 2812 |
| Coptic calendar | 37–38 |
| Discordian calendar | 1487 |
| Ethiopian calendar | 313–314 |
| Hebrew calendar | 4081–4082 |
| Hindu calendars | |
| - Vikram Samvat | 377–378 |
| - Shaka Samvat | 242–243 |
| - Kali Yuga | 3421–3422 |
| Holocene calendar | 10321 |
| Iranian calendar | 301 BP – 300 BP |
| Islamic calendar | 310 BH – 309 BH |
| Javanese calendar | 202–203 |
| Julian calendar | 321 CCCXXI |
| Korean calendar | 2654 |
| Minguo calendar | 1591 before ROC 民前1591年 |
| Nanakshahi calendar | −1147 |
| Seleucid era | 632/633 AG |
| Thai solar calendar | 863–864 |
| Tibetan calendar | 阳金龙年 (male Iron-Dragon) 447 or 66 or −706 — to — 阴金蛇年 (female Iron-Snake) 448 or 67 or −705 |
