Son algunos días del calendario escolar en los cuales los niños no tienen clases, pero los maestros se dedican a hacer cursos y talleres, como parte de su labor.

--------------------------------------------------
Note added at 16 mins (2020-05-08 01:53:38 GMT)
--------------------------------------------------

La verdad es que en principio me parecerían sinónimos.
Encontré lo siguiente en www.pcs.k12.va.us:
Teacher Professional Development Day (required) This day is targeted annually to not be a student day since majority of our schools are used as Election precincts. For safety reasons, no students are in schools during this day since there are a large number of people in and out of the buildings.
Two days are used for the break between semesters (one day to be used as a Teacher Professional Day). It is extremely important to high schools and some middle school block courses since they are changing to a new group of students and courses. Guidance will be busy making student schedule adjustments. This is basically starting school again for these teachers and students with new rosters, distribution of books, classroom rules and requirements, etc.
Sin embargo, parecería que en tu caso están usando las dos expresiones exactamente al revés (aunque la elección sea el 3 de noviembre, no el 30 de octubre).

--------------------------------------------------
Note added at 17 mins (2020-05-08 01:54:28 GMT)
--------------------------------------------------

A ver si alguien sabría algo más del tema.