Final answer:
The President likely executed a pocket veto if he did not sign a bill within the ten-day window when Congress also adjourned, as the bill does not become law without his signature in that scenario.
Step-by-step explanation:
If by December 30 the President has not signed a bill that was sent to him by Congress on December 24, he has probably executed a pocket veto. The President has ten days (excluding Sundays) to sign a bill into law, veto it with objections, or take no action and allow it to become law without a signature. However, if Congress adjourns during that ten-day period and the President takes no action, the bill does not become a law, which is known as a pocket veto. This is based on the constitutional provision outlined in Section 7 Clause 2 along with the legislative powers of the President.