The Kohen hedyot, ordinary Kohen, brought his Korban Tamid, continual offering, once upon his induction into the avodah, service. The Kohen Gadol, High Priest, brought it daily: half of the tenth-eifah in the morning; and half in the evening. The Mishnah in Menachos (50b) teaches that the tenth-eifah is split in half, with half brought in the morning and the other half in the evening. It is required that the two halves are from the same whole tenth-eifah. The Kohen may not use a half that he had at home. The Rishonim debate whether the twelve breads that are used must each be halved, or six in the morning and six in the evening suffice. In any event, explains Horav Gamliel Rabinowitz, Shlita, we derive from here that a requisite for this korban is that it come from a whole portion split in half. If one of the halves became tamei, ritually contaminated, the Kohen must bring a new whole tenth eifah and split it in half.
This teaches us that at all times, under all circumstances, whether it is day – when the sun is shining and life is good – or it is night – gloomy, plagued with adversity – he must provide a full/whole service. One does not serve Hashem only when things are going his way, when life appears to be filled with good fortune. We begin with a whole and split it – leaving a half for later, in the evening. We must always be cognizant of the challenges that we might confront. Likewise, when we are faced with adversity, we do not despair of salvation. The “day” follows the “night.” They are not two separate halves, but two parts of one whole. All too often, when we feel that we have been handed a bitter pill, we lose sight of what tomorrow may bring. Likewise, when the sun is shining, we forget that our good fortune can quickly change. The continual-offering that the Kohen Gadol brings reminds us that serving Hashem is continual, in all times and in all circumstances.