Constantinople and Moscow keep the Nativity on the same day and celebrate it a fortnight apart. Both say 25 December; one reads that off the Revised Julian calendar, which has agreed with the Gregorian since 1600 and will until 2800, and the other off the unreformed Julian, which now lags thirteen days and will lag fourteen from 2100. The lag is not doctrinal and not a difference of feast — it is the accumulated error of a leap-year rule that added a day too often, corrected in the West in 1582 and not in the East. Pascha is a separate matter again: almost every Orthodox church, New Calendar or Old, still computes it on the Julian reckoning.
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