If I want the Hebrew date for a Julian date (e.g., a date in the 15th C CE) is it better if I convert it to what it would have been in the Gregorian calendar, and then find the Hebrew date? Thanks.

I'm surprised that Shabbat Shirah is not included among "special shabbatot." Also, I wouldn't exactly call Sigd a "modern holiday." That's a very "Ashkenazi-centric" idea! Thanks