You must use capital letters in the following instances, particularly in formal writing: 1) In the names of people, places, days and months (not seasons), languages, religions, and so on (also known as "proper nouns"); 2) At the beginning of a sentence; 3) In titles of books, films, organizations, and special days and holidays; 4) In abbreviations or acronyms; 5) In titles that come before names; 6) And in compass points when they indicate regions, among other instances.
Therefore, the following words in your examples should be capitalized:
11. In the West, the weather is more moderate year round than it is in the Midwest, but Northern California is cooler and more humid than Southern California.
12. Alan Anderson, Treasurer of the Roosevelt School PTA, endorsed the purchase of a computer for the school library, but PTA President Sue Jenkins has reservations about making such a purchase now.
13. Our family has its spring reunion on Memorial Day Weekend each year.
14. We missed the Sixteenth Ward political rally held by Alderman Paul T. Johnson.
15. In Northern Michigan, the autumn leaves are spectacular in mid-October.
