'in next month' VERSUS 'on next month'

[Bobo]

English Language Tests, Intermediate level

ESL/EFL Test #206 "Prepositions of Time and Date (4)", question 4

......... next month, I'm expecting my family to visit from the Virgin Islands.

(a) On
(b) (none)
(c) In

English Language Tests, Intermediate level

ESL/EFL Test #206 "Prepositions of Time and Date (4)", answer 4

Next month, I'm expecting my family to visit from the Virgin Islands.

Correct answer: (b) (none)
_________________________

When can I use IN or ON?

Bobo

[Mister Micawber]

.
We do not usually use in/on before this, that, next, last.

In February, in 2007, in (the) spring.
On Tuesday, on weekdays, on January 31st.
.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber