There are clear rules in transforming singular nouns to plural nouns. Below are the rules that involve words ending in y.
Nouns ending in y preceded by a vowel form their plurals by only adding -s. For example, we have the words:
boy-boys
key-keys
day-days
valley-valleys
joy-joys
However, it a noun ending in y is preceded by a consonant, the y is changed to i and -es is added. For example, we have the words:
baby-babies
lady-ladies
calamity-calamities
ceremony-ceremonies
butterfly-butterflies