Why would Equestrians use our calendar names? (With some worldbuilding)

(Parts of this post were posted as part of comment >>9996435 on https://www.fimfiction.net/story/418292/22/twilight-gets-a-puppy-season-2/it-crosses-your-path-nightmare-night-bonus-episode)

I have noticed that many writers have Equestria use our Gregorian calendar, including the English-language names of the months and days. I started wondering: Why? Aren't some of them based on people and things from Earth (henceforth Terran to avoid confusion with Earth ponies) that Equestrians wouldn't get?
(Note that this doesn't include stories like Hands or Universal Acceptance, in which ponies were created and/or heavily influenced by humans)

Let's go through the month names in this post since days of the week are explicitly mentioned in the show itself and the months are more interesting for worldbuilding.

Two of them, January and July, are ones that the Equestrians probably shouldn't use because they are named after Terran people or deities (respectively: Probably the Roman god Janus but possibly the similarly-named Roman goddess Juno; and the Roman general and would-be autocrat Julius Caesar).
August is also known to have been named after a person, but Augustus was a title bestowed upon him by the Roman Senate (his full name at this point was Imperator Caesar Divi Filius Augustus, which essentially translates to "General Caesar, Son of the Divine, the Majestic"; imagine if the British Parliament changed Queen Elizabeth's name to Queen Elizabeth Daughter-of-God Majesty...). Since "august" is actually an English word, and since I have, while reading stories on this site, actually come across a pony seriously called Queen Majesty, I am going to have to say that August is a plausible name for an Equestrian month.

As for January and July, maybe you could finagle them as Griffon in origin, since they start with J and you could say that maybe they originally started with G and became J in transliteration?

Of the nine other months:

February is named after a Terran festival. The Terran festival in turn is named after blood sacrifices, but the word ultimately descends from a Proto-Indo-European root meaning "to burn", "to warm", "warm", or "hot" (hence also the term febrile, "feverish"), so I don't think that the blood sacrifices part is necessary.
I could imagine a pony Februa as an end-of-winter ritual, from before industrialised weather control, or as a festival to celebrate the first of the year's estrus periods (this latter of course assumes that ponies have estrus periods, and to a lesser extent that they have spring estrus rather than summer estrus). If it was the former, then it was evidently displaced by Winter Wrap-Up or perhaps, if it had elements of both, split into two festivals (ie, WWU and Hearts and Hooves Day).
If Equestria has a month by this name, I can very strongly see February as the first month of spring, although it could also easily be a summer month if story reasons dictate that Hearts and Hooves Day be in the summer, given how strongly people associate H&H Day with Valentine's Day*.

March is named after the Roman deity Mars but could easily have a similar-sounding name in Equestria by chance: The word also means a form of travel and borderlands, and is related to the word marker, as in boundary marker.
The difficulty here is that March could refer to so many things, depending on what an individual author wants, that it is hard to say definitively what the Equestrian version of March would be named after.
Examples: Maybe the month commemorates a month-long pilgrimage or military campaign. Maybe it used to be the time when ponies traditionally reasserted and "marked" the edges of their territories after winter. Maybe Earth ponies used to worship a deity called Watchful March and there was a festival in his honour in what is now March.

April may be named after the season of Spring (the term is traditionally said to derive from the Latin word aperire, "to open", which refers to the opening of spring-blooming flowers and the newly-regrown leaves of deciduous trees, the same way some people refer to Autumn as Fall because of the falling leaves of deciduous trees) but could be named after the Greek goddess Aphrodite (her Etruscan name was Apru).
I am going to say that a month with this name is quite plausible, and would very likely be a spring or summer month. Further, if pony estrus is in late spring and estrus is in any way socially important, I would suggest that ponies would have a historical or current estrus celebration in April (if something is socially important, polytheists will make a festival and/or deity out of it).

May and June are probably named after the nowadays-obscure (do you know the names of the Pleiades?) Greek deity Maia and the aforementioned Juno but could have been (as the Roman poet Ovid hypothesised) named after the words for elders and younger people (maiores and iuniores respectively).
This one is harder for me to imagine, but maybe the months could be named after two different ceremonies, one celebrating the newly-adult and one celebrating young colts and fillies. This is especially the case if most or all childbirths happen at the same time of year, as would happen in a milleu with seasonal fertility cycles.
If most foals are born in February, I can easily imagine June hosting a celebration of foals surviving the first 3- to 4-month period, from a time when infant mortality was far higher than it is now and therefore foals weren't considered truly ponies until a certain age because they were too likely to die (under ancient Jewish law, newborn babies were treated more or less the same way).
As for May, I can imagine a coming-of-age ceremony. However, I cannot see any particular reason why May and June would need to be adjacent to have the maiores and iuniores etymologies.
I would suggest that Equestrians may or may not use those names, and that even if they did, they need not be adjacent months except for reader convenience.

September through December are named after the Latin words for 7 through 10, and therefore are perfectly reasonable if you take it as a given and/or translation convention that ancient equine language be represented by Latin, and if you take it as a given that the ponies left a third or more of their months unnamed (not much of an ask, given that this is not actually that rare in Terran calendars).
However, it could be too much of a contrived coincidence for months labelled 7 through 10 to also be the 9th through 12th months just like the Terran versions of those months.
Also, if Equestrians were to use the normal Gregorian calendar, Hearthswarming would be in December (or maybe November) and this event is the foundation of Equestria. Why would Equestrians leave this month numbered rather than named?
In my view, Equestrians might use some or all of these four names, but December is a little implausible unless Hearthswarming is taken as being celebrated in their equivalent of January.

* February, especially in its Roman name (mensis Februarius or February month), more or less means "the month with V-Day in it".
Strictly speaking, the name refers to the Roman festival of Februa (AKA Lupercalia) of which Valentine's Day is the closest Christian equivalent, due to its modern association with (an aspect of) sex.
However, the resemblance of Februata to either the modern or historical V-Day, or for that matter between the modern and historical V-Days, is highly overstated: The original Valentine's Day was not sexual or romantic at all, and the first evidence of its romanticisation is from Chaucer in 1382's Parlement of Foules, in a passage about the mating habits of birds; whereas Februa was more of a fertility festival than a romantic one and the most similar aspect to the modern V-Day, Juno Februa, may have been a retroactive imposition of the modern V-Day onto the ancient Romans.