Leviticus 16:3

This shall be the rite (mida) for the entering of Aharon into the holy place. With a young bullock, having no mixture, for the sin offering, and a ram for the burnt offering.

with a bullock This does not mean that he should bring the bullock into the Holy of Holies. Rather, it means that he must first sacrifice his own bullock as a sin-offering, to atone for himself and for the kohanim (some say that his bullock also atones for the Levites, because they came across the verse “You will inscribe Aaron’s name upon the rod of the Levite tribe” [Numbers 17:18] — but this is far-fetched; the Levites are reckoned with the rest of Israel).

The secret of ‘b’zoth’ (With this) shall Aaron come into the holy place, is analogous to the secret of ‘zoth’ (this is) the token of the covenant which I have established. I have already explained it. Our Rabbis alluded to it in saying in Vayikra Rabbah: “Rabbi Yudan explained the verse [‘With this’ shall Aaron come into the holy place] with reference to the High Priest when entering the Holy of Holies: he came with many bundles of commandments [fulfilled by Israel] in his possession. He came in there by merit of [their studying] Torah, as it is said, and ‘this’ is the law; by merit of circumcision, as it is said, ‘This’ is My covenant, which ye shall keep; by merit of keeping the Sabbath, as it is said, Happy is the man that doeth ‘this’ [… that keepeth the Sabbath]; by the guarding influence of Jerusalem, as it is said, ‘This’ is Jerusalem; by the guarding influence of the [twelve] tribes, as it is said, and ‘this’ is that their father spoke unto them; by the guarding influence of Judah, as it is said, And ‘this’ [Moses said] for Judah; by the guarding influence of the congregation of Israel, as it is said, ‘This’ thy stature is like to a palm-tree; by merit of [Israel having observed the law of] the heave-offering, as it is said, And ‘this’ is ‘ha’terumah’ (the offering); is here understood in the wider sense of “something set apart.” The portion given to the priest from the produce of the corn, etc. is therefore terumah. In this particular verse it refers to that which was set apart as a freewill gift for the erection of the Tabernacle. by the merit of [Israel giving] the tithes, as it is said, and try Me now with ‘this;’. by the merit of bringing the offerings, as it is said, With ‘this’ shall Aaron come.” This text requires a lengthy exposition, but it is all explained in our commentary [in various places].

בזאת יבא...בפר בן בקר, by sanctifying the bull as a sin offering, and the ram as a burnt offering, and by donning the appropriate priestly garments, the linen ones which had no decorative additions and consisted only of white linen. On the one hand, that High Priest was not to enter the sanctuary until after the burnt offering had been sacrificed, but he was allowed to enter for the purpose of offering the incense as soon as the sin offering had been slaughtered.

בזאת, “herewith, etc.” the numerical value of the word בזאת is 410, a reminder of the number of years Solomon’s Temple was operative before the last two tribes went into exile.