The notice of effectiveness is a public declaration by the Securities and Exchange Commission that a public company’s registration statement has been accepted.
For shares in a public company to trade on the open market they must be registered by the company. These registration statements are reviewed by the Commission for a period of time before approval is given, and this notice marks the successful completion of this review process.
when an company comes public it becomes effective. So for instance last week Vonage would have become effective on the first day of trading.Source(s): my experience