An infected tick must be attached for 24 to 48 hours to transmit Lyme disease (and at least 12 to 24 hours to transmit anaplasmosis, another serious tick-borne disease). Lyme disease is unfortunately a fairly common canine disease. Symptoms can include fever, reduced energy, and lameness.
D-day(n.) (Mil.) the day designated for the beginning of a planned attack by a military force; in communications the day is often otherwise unspecified for security reasons; as, the day before D-day the troops will move into position.
The days in nowadays, etc. is a relic of the Old English and Middle English use of the adverbial genitive. 1918, date set for the beginning of a military operation, with D as an abbreviation of day, cf. H-hour, also from the same military order of Sept. 7, 1918: