"In the afternoon"
Maybe it's used like that in some dialects, but it's too dialectal to be an accepted answer. You could easily be misunderstood if you used that instead of på eftermiddagen.
On the other hand om eftermiddagarna works in Standard Swedish for talking about things you do every afternoon, but that would be 'in the afternoons' in English.
Different languages use different prepositions to express things. So you just have to learn that på eftermiddagen corresponds to in the afternoon in English and i eftermiddags (note the correction) corresponds to this afternoon. Another example is i två år which is for two years.
I wasn’t referring to grammatically past tense. I was referring to that ”this afternoon” can mean both a time in the past and the future.
- Did you see the game this afternoon?
- Are you going to see the game this afternoon?
In Swedish these are different: i eftermiddags and i eftermiddag respectively.
Not just in Sweden. Most western cultures historically had the major meal of the day in the middle of the work day because a much larger percentage of people worked outside, and high noon is not exactly when you want to be doing manual labor in many climates. English has seen this shift too, as 'dinner' used to be the big mid-day meal, while 'supper' was the smaller late evening meal.
The shift in general is probably related to the fact that it was not unusual in some parts of Europe for more 'white collar' jobs (such as publicans or lawyers) tending to eat only two meals during the day, usually late morning before they started their duties for the day and then late afternoon or early evening after they were done working for the day, with the late meal typically being their big meal for the day.