'on next week' versus 'in next week'

[Handira]

English Language Tests, Intermediate level

ESL/EFL Test #203 "Prepositions of Time and Date (1)", question 6

I have to go for a check-up ......... next week. I go for a check-up once a year.

(a) about
(b) on
(c) (none)

English Language Tests, Intermediate level

ESL/EFL Test #203 "Prepositions of Time and Date (1)", answer 6

I have to go for a check-up next week. I go for a check-up once a year.

Correct answer: (c) (none)
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May i know what is the different btw in and on?

Handira

[Mister Micawber]

.
Briefly--

On a day: on Tuesday; on January 31st.
In a month, year, etc: in January, in 2006, in the 21st century.
.
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Native English teacher at Mister Micawber's