Explanation:

They always give the available seats to whom comes first.
Whomever is an object pronoun and works like the pronouns him, her, and them (Give the document to whomever in the department).
Whoever is a subject pronoun and works like the pronouns he, she, and they (Whoever wrote this poem should win a prize).
Whom should be used to refer to the object of a verb or preposition. When in doubt, try this simple trick: If you can replace the word with “he”' or “'she,”
Whichever' means 'any one from a limited set'. Always choose 'whichever' before words like 'one', 'day', etc. You can come on whichever day you wish. You can choose whichever one you want.