(Bamidbar 28:11) "And in the beginnings of your months, you shall offer a burnt-offering to the L-rd": What is the intent of this? From (Ibid. 4) "The one lamb shall you offer in the morning," I understand the beginning of the month (Rosh Chodesh) also to be included; therefore, Scripture removes it from the general category for greater stringency — the mussafim (of Rosh Chodesh). (But why is a special verse needed for the mussafim of Rosh Chodesh?) Let it be derived from (the mussafim of) Sabbath? (In that case I would say:) Just as (the mussafim of) Sabbath are two lambs, so, those of Rosh Chodesh; it is, therefore, written "And in the beginning of your months, you shall offer a burnt-offering to the L-rd, etc." — Scripture adduces a different numeration. R. Elazar b. R. Tzaddok says: There was a hin (a liquid measure) in the Temple, with markings: Thus far (the libation) for a bullock; thus far for a ram; thus far for a sheep. R. Yishmael says: There was no hin, but an additional measure of a log and a half in which was apportioned the high-priest's meal-offering, a log and a half in the evening, a log and a half in the morning.
Sifrei on Numbers
Sifrei Bamidbar 145
(Bamidbar 28:14) "This is the burnt-offering of the month in its month." And not the burnt-offering of this month for a different month. If one did not sacrifice it this month, I might think he could sacrifice it on a different month. It is, therefore, written "This is the burnt-offering of the month in its month," whereby we are taught that if the day passes, its sacrifice "passes." "for the months of the year": What is the intent of this? From "And the beginnings of your months you shall offer a burnt-offering to the L-rd," I might think the minimum of months — two; it is, therefore, written "for the months of the year."
(Bamidbar 28:15) "And one kid of goats as a sin-offering to the L-rd": to atone for "a grave of the depth" (i.e., tumah which was discovered only after one had come in contact with it.) "in addition to the continual burnt-offering shall it be offered and its libation.": We are hereby apprised that the mussafim (of Rosh Chodesh) are sacrificed only between (the two temidim).