NSF had released a Dear Colleague Letter announcing that, effective August 15th, NSF's Directorate of Engineering (CBET, CMMI, ECCS, and EEC) will no longer have proposal windows or target dates for core (investigator-initiated) proposals to their disciplinary programs. This does not apply to targeted solicitations such as CAREER, MRI, ERC, etc. As a part of this change, they will also implement a guideline in which a declined proposal (or substantively similar proposal/topic by the same PI) is ineligible for resubmission until a minimum of one year has passed from the date of its initial submission.
While NSF states that their main motivation is to improve the quality of the proposals they receive since fewer proposals will be submitted before they're ready, it's also true that when there is no official deadline, NSF programs receive fewer proposals. As NSF has been struggling with high proposal pressure, this effect was likely another important motivation for this change.
As we all know, something that can be done any time is often done at no time. Don't let this happen to you! Set your own deadline for submitting your core proposal, write it in big red letters on your calendar, announce it to the world - whatever it takes to generate the feeling of urgency that you and your co-PIs need to get your proposal done!
(See NSF's FAQs about this policy change here.)