I'd hyphenate mid-August.
See this: Dr. Grammar's Frequently Asked Questions Page
Mid- or just Mid?
"In forming compounds, mid- is normally joined to the following word or element without a space or hyphen: midpoint. However, if the second element begins with a capital letter, it is separated with a hyphen: mid-May. It is always acceptable to separate the elements with a hyphen to prevent possible confusion with another form, as, for example, to distinguish mid-den (the middle of a den) from the word midden. The adjective mid is a separate word, and as is the case with any adjective, it may be joined to another word with a hyphen when used as a unit modifier: in the mid Pacific but a mid-Pacific Island" (The American Heritage Dictionary of the English Language)
or, my favorite reference, the OWL at Purdue:
Use a hyphen with the prefixes ex- (meaning former), self-, all-; with the suffix -elect; between a prefix and a capitalized word; and with figures or letters:
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.