What does Boxing Day have to do with boxing?
1727, "resemblance of sounds between words," from French assonance, from assonant, from Latin assonantem (nominative assonans), present participle of assonare "to resound, respond to," from ad- "to" (see ad-) + sonare "to sound" (see sonata). Properly, in prosody, "rhyming of accented vowels, but not consonants" (1823).
in prosody, repetition of stressed vowel sounds within words with different end consonants, as in the phrase "quite like." It is unlike rhyme, in which initial consonants differ but both vowel and end-consonant sounds are identical, as in the phrase "quite right." Many common phrases, such as "mad as a hatter," "free as a breeze," or "high as a kite," owe their appeal to assonance. As a poetic device, internal assonance is usually combined with alliteration (repetition of initial consonant sounds) and consonance (repetition of end or medial consonant sounds) to enrich the texture of the poetic line. Sometimes a single vowel sound is repeated, as in the opening line of Thomas Hood's "Autumn": I saw old Autumn in the misty morn