From JewishGen, the JOS Calendar Converter can convert a civil (Gregorian calendar) date into the equivalent date on the Hebrew calendar, and vice versa. It can also display Yahrzeit dates for consecutive years.
This is an interactive German church calendar. Select the year and it will tell you one what days various feast days occurred. Sometimes events in church records are stated by the feast day (or number of Sundays after a feast day). This website will help determine the exact calendar date.