The usually celebrated date of the Unification of Bulgaria with Eastern Rumelia is September 6th because this is the original date in the Old Style. If someone wonders why the difference is 12 rather than 13 days, it turns out that for dates in the 19th century the correction is 12 days and for dates in the 20th century - 13 days. This is because years like 1700, 1800 and 1900 are not leap, but 1600 and 2000 are leap. Could it be more confusing?!