Indeed, remembering the Giving of the Torah is itself a mitzvah of the Torah: “Only beware for yourself and guard your soul exceedingly, lest you forget the things your eyes saw... the day that you stood before Hashem your G-d at Chorev” (Devarim 4:9–10). This is the mitzvah of remembering the event at Sinai. Ramban counts it among the 248 positive commandments. Although the Rambam does not include it in his enumeration of the mitzvos, he nevertheless holds that it is an absolute obligation, as he writes in Iggeres Teiman. See there.
In truth, the Exodus of Egypt is the beginning and root of accepting the yoke of Heaven. The mitzvah of remembering the Exodus of Egypt reminds us that Hakadosh Baruch Hu brought us from slavery to freedom in order that we become His people, meaning His servants, instead of being servants of Pharaoh in Egypt. It keeps us always remembering and acknowledging His kingship in the world and the yoke of His sovereignty that rests upon us.
The completion of accepting the yoke of Heaven took place at the Giving of the Torah at Sinai as a continuation of the Exodus of Egypt, as the Sages said: “It is analogous to a king who entered a province. His servants said to him: ‘Issue decrees upon them.’ He replied, ‘When they accept my kingship, I will issue decrees. For if they do not accept my kingship, they will not accept my decrees.’ So did the Omnipresent say to the Jewish people: ‘I am Hashem your G-d, Who brought you out of the land of Egypt, from the house of bondage. Did you accept My kingship in Egypt?’ They replied: ‘Yes.’ He said to them: ‘Just as you accepted My kingship, so too accept My decrees: You shall have no other gods before Me.’ (Mechilta, parashas Bachodesh, Parashah 6). Thus, remembering the Exodus of Egypt connects us to the foundation and root of accepting the yoke of Heaven, while remembering the Giving of the Torah at Sinai connects us to its completion and actual implementation.
