At this time, there is no data that suggests when exactly these frost-free days will be distributed throughout the year, so it is likely best to us your own judgement and observations to make decisions on this. We do know, though, that apples and many fruits and flowers are blooming earlier, and cover crops are able to be planted later in many regions. This may indicate that frost-free days are being extended into both the Spring and Fall seasons, but exact numbers are unavailable.
Also, it is important to note, that although the general trend tends toward more frost-free days, late/early frosts can still occur, so preparation for these is still vital.
I hope this answers your question well, and let us know if you have any other inquiries.