Not being Jewish I scheduled an event this year, only to learn shortly before it occurred that the date was a Jewish Holiday and so I had essentially excluded Jews. I live in the United States, and it is impossible for a non-Christian to not be aware of Christian holidays. Jewish holidays are not commercialized (at least to my notice).
In the future when scheduling events how can I know what dates to avoid if I don't want to accidentally exclude Jews?
HebCal is a full Hebrew calendar that provides lists of all the Jewish holidays, for any year.
In the interest of full disclosure, i have done volunteer programming for them, but am not officially affiliated.
Here is a list of all the Jewish holidays for the year 5775 (this year). Note that not every item on that list is a full holiday where work is forbidden. The ones in the first section ("major holidays") that are in bold are the dates to be sure to avoid.
If you use MS Outlook you can go into the calendar options (The menu has changed throughout the versions, so look up the Help Screen for the exact procedure) and add the Jewish Holidays to your calendar.
One thing it won't tell you, though, is which holidays are days where work is forbidden. So here's the short list (assuming you are concerned for those outside Israel):
Be aware that some Jews have various customs regarding the types of activities they will do during the 3rd-6th days of Passover and during the 3rd - 7th days of Tabernacles. If you're planning an event, try to get a "consensus" from your guests.
The College Journal of Mathematics has an article Converting Between Dates in the Hebrew and Roman Calendars by Agus, Conway, and Slusky.
It provides a pretty straightforward method that can be done with pen and paper.
