: Generally within 30 days of the marriage on a group (or at a later open enrollment if you miss the date cut-off). Group coverage allows a very few "triggering events" for coverage outside of the normal open enrollment period, and a new marriage is one of those events. (Loss of medical coverage previously available from a spouses group plan is another trigger.) The spouse and step-children (if they live with you and have become your IRS dependents) can be added within 30 days of the event. If you miss that window, you will generally need to wait until open enrollment. Do not let your spouse drop any existing coverage until approved on the new plan.
Individual coverage: State laws vary for individual coverage, but you can apply immediately after the marriage. Medical underwriting may be required for individual coverage in some States. Prior to marriage, add to your "to do" list a new analysis of your coverage, and your future spouses coverage, and know in advance which plan will best suit your growing family.