I see Haruo explained that leap year thing.

I checked with Ecclesiastical Calendar (see Mark, above) any way since I know it to be a very careful and thorough (and arithmetic) site on this stuff. Marcos Montes there concurs: 1800 was a leap year in Julian Calendar, not in Gregorian, so another day dropped.

1900 was a leap year in Julian, not in Gregorian, so another
day dropped.

2000 is a leap year in both calendars, so no extra day dropped.

The next day dropped will be in 2100, which is a leap year in
the Julian calendar, but not in the Gregorian."

He also gives some good info in the distinction between "Old Calendarists" who use the Julian Calendar to determine the date of ALL religious feasts...and the "New Calendarists" who use the Julian Calendar for movables (eg Easter) but Gregorian for fixed celebrations.

I think I'll go home and celebrate Yap Constitution Day today and Haym Salomon Day on Jan 6th and the Japanese Bullfinch Festival on Jan 7th and Chou En-Lai Day on the 8th. That will do it for the year. (Many Asians use varying but related lunar calendars AND Gregorian calendars for other things.) The notion of using different calendars simultaneously isn't unusual.