SIMPLE IRA, SIMPLE 401(k) or Safe Harbor 401(k) is October 1.
Solo 401(k) plans is December 31st.
If retirement planning is important to you, and, of course, it should be, keep in mind that you need to be working on getting the plans created for 2015 funding.
SEP retirement plans can be set up at anytime prior to the filing of your return for the prior year. So if you wanted to make a 2014 SEP contribution, you still have time to set one up and make a contribution so long as you have an extended return that you have not yet filed.
Start thinking about your future and get tax savings today by setting up a retirement plan for you and your employees.