I have a Jewish friend who has mentioned that "in the past," Easter was used as a time for Christians to persecute Jews, for their role in killing Jesus. I know that a lot of tragedies occurred, and "the Jews in Europe" can't be treated as a monolithic group, but as someone wanting to learn more about this history:

1. Was this a widespread occurrence or was it limited to a few places / time periods
2. Did it occur on many holy days (e.g. ash wednesday, christmas, etc.,) or was it specific to Easter?
3. Was there any element of "identifying" who was a Jew and who was Christian -- for example, was the Ash Wednesday practice of wearing ashes on one's forehead ever used to identify Jewish people?