According to Cambridge Dictionary,
with a singular day of the week to refer to one occasion:
I’ve got to go to London on Friday.
with a plural day of the week to refer to repeated events:
The office is closed on Fridays. (every Friday) In informal situations, we often leave out on before plural days:
Do you work Saturdays?
I’ve got to go to London on Friday.=I’ve got to go to London this/next Friday.
The office is closed on Fridays.=The office is closed every Friday.
However, I would think that "in July" or "in Winter" can be used for both cases "one occasion" & "repeated events", but I am not so sure.
- I’ve got to go to London in July.=I’ve got to go to London this July.
- The office is closed in July.=The office is closed every July.
- I’ve got to go to London in Winter.=I’ve got to go to London this Winter.
- The office is closed in Winter.=The office is closed every Winter.
We have "on Monday" for 1 occasion & "on Mondays" for repeated events, can we have similar patterns for "in July" or "in Winter"?
For example, can we say the following?
- The office is closed in Winters / in Julies.=The office is closed every Winter / every July.