19th day of the month
Refers specifically to the 19th calendar day. Used with dates, schedules, and deadlines.
The meeting will be held on the 19th.
19 days
Indicates a duration of nineteen days. Often used with counters or in expressions of time span.
This project took 19 days.
Archaic slang; not used in modern Japanese. The connection to the number 19 is unclear and may be based on old wordplay.
In Edo-period slang, １９日 apparently referred to a fool.
The meeting is on the 19th.
It took 19 days for the construction to finish.
Please submit the report by the 19th.
I'll be traveling for three days starting on the 19th.