The spelling of the word "Lunday" can be explained through the use of IPA phonetic transcription. The word is pronounced as /ˈlʌndeɪ/, with the stress on the first syllable "Lun". The "u" is pronounced as the short vowel sound /ʌ/, while the "a" is pronounced as the long vowel sound /eɪ/. The "d" is voiced, and the final "ay" sound is formed by the combination of the letters "a" and "y". Overall, the spelling of "Lunday" accurately reflects its pronunciation.