Most nouns form the plural by adding -s or -es. For example, boat becomes boats, or hat becomes hats.
A noun ending with a consonant and the letter y forms the plural by adding -ies. For instance, city becomes cities, and baby becomes babies.
This is not true for all nouns. There are many irregular plural nouns, such as woman (women), potato (potatoes), or tooth (teeth), and some nouns are the same in both the singular and plural forms (such as sheep and fish), but most nouns are made plural by adding -s, -es or -ies.
When we want to show that something belongs to somebody or something, we usually add ’s to a singular noun, and an apostrophe ‘ to a plural noun. For example, the boy’s ball (one boy) or the boys’ ball (two boys). The number of balls doesn’t matter, only the number of possessors (in this case, boys).
We often use possessive ’s with proper nouns (names): Mary’s car, Sarah’s son, or Robert’s book. If the name ends in s, like Charles, we usually treat it like a singular noun and add ’s: Charles’s friend. However, it is also correct to just add the apostrophe: Charles’ friend.
So, when Brian talks about giving valentines, he is talking about giving a special kind of card. But when he talks about Valentine’s Day, he is talking about the day that is named after Saint Valentine.