Most people know that they have until the first individual federal tax filing deadline, not including extensions (usually April 15), to make a traditional or Roth IRA contribution for the prior tax year, unless that day is a Saturday, Sunday or federal holiday. Since the term "federal holiday" includes any legal holiday observed in the District of Columbia, the final deadline to make a 2015 traditional or Roth IRA contribution this year is April 18, 2016 because the District of Columbia observes Emancipation Day on Friday, April 15, 2016.
Traditional and Roth IRA contributions for the prior-year may be accepted by an IRA custodian if postmarked by the deadline. But the last thing you want is to not make your IRA contribution timely due to a U.S. Post Office "error" in properly postmarking your envelope. SEP-IRA contributions on the other hand, can be made as late as the due date (including extensions) of the business entity sponsoring the SEP-IRA.