Add Get The Names Of The Month as a Ruby TIL

ruby/get-the-names-of-the-month.md

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+# Get The Names Of The Month
+
+Ruby's `Date` object has a `MONTHNAMES` constant that returns an array of names
+of the month. You'd think that means the array contains 12 items. However, the
+size of that array is 13.
+
+```ruby
+> Date::MONTHNAMES
+=> [nil, "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]
+```
+
+Notice it has all 12 months, plus an initial value of `nil`.
+
+This is because it allows us to more intuitive access a month by it's index
+without having to do a little subtraction. If I want to know what the 9th month
+is, I can do an array access for `9`.
+
+```ruby
+> Date::MONTHNAMES[9]
+=> "September"
+```
+
+Because arrays in Ruby use 0-based indexing, without this baked in `nil` value,
+you'd instead get `"October"` when passing in `9`.