Must contributions to all retirement plans -- IRAs, SEPs, and Keoghs -- be made by December 31, or do you have until April 15th?
None of the retirement plans you mentioned requires that contributions be made by the end of the year.
An IRA contribution can be made all the way up until April 15 after the end of the year for which you want to make a contribution. SEP and Keogh contributions can be made up to the due date for filing your tax return -- or the extended due date, if you've requested an extension for filing your return.
Keogh plans must be set up by December 31, even if you delay making your contribution until later.
Both SEPs and IRAs can be established after the end of the year. In fact, the deadline for establishing an IRA or a SEP is the same as the contribution deadline: April 15.