The account in Genesis of the seven days of creation–And the evening and the morning was the first day–is, I assume, the reason Jews celebrate a day as starting at sundown and ending on the next day. I would have thought that some fundie sect of Christianity that believes in the literal truth of the bible, would have adopted this understanding. Perhaps celebrated Sabbath, from sundown on Saturday to sundown on Sunday. But I can find nothing on the web that suggests there is such a sect.
My recollection is that the liturgical day in the Roman Catholic church and the Anglican communion starts at sundown.
Right, and hence you can meet your obligation for Sunday mass by going on Saturday evening.
Though in practice, that’s usually stretched a bit, such that a Saturday evening mass might be as early as 4:30 or 5:00, even in the summer when that’s long before sunset.
Prior to 1968, Saudi Arabia reset their clocks to 12 AM every day at sunset.
Here’s a Wikipedia article on 7th day churches. They seem to not only have the “Sabbath” on Saturday but deem it lasting from sunset Friday to sunset Saturday. What I know about 7th Day Adventists is that this “day” start is purely religious and they generally go along with the local time and calendar otherwise.
Note: So many Christians call Sunday “the Sabbath” in the US but is not officially the Sabbath in historical Christianity. (In fact calling yourself a Sabbath Keeper in olden times got yourself in a ton of trouble.) So I go with “Sabbath” for Saturday and “The Lord’s Day” for Sunday.
This is correct. As to your question, I dunno.
Orthodox Church is also sundown to sundown. Due to some of the church being very far north… there is a 6pm to 6 pm tradition.
I was alwas curious why Pope Gregory (I forget his number) who finalized the calendar that bears his name didn’t make Sunday the seventh day. Any Christian would know that the sabbath is supoosed to be on the seventh day. In his calendar, the seventh day is Saturday.
Because the Resurrection occurred on Sunday, so that’s the first day of the week.
John 10:1, 2
20 Early on the first day of the week, while it was still dark, Mary Magdalene went to the tomb and saw that the stone had been removed from the entrance. 2 So she came running to Simon Peter and the other disciple, the one Jesus loved, and said, “They have taken the Lord out of the tomb, and we don’t know where they have put him!”
See also @ftg 's post, above.
The argument made by my local diocese growing up was “a time that is after sunset on any day of the year counts” so Saturday evening mass could happen hours before sunset in the summer so long as that same clock time was after dark in the dead of winter.
Really the argument is, “staying out late Saturday night and sleeping in Sunday morning rocks, so lets get the stuff I have to do in that timeframe out of the way as early as possible”.
I imagine that the Reykjavik diocese gets away with a lot. 
A sect? A lot of Medieval Europe used to do that, not just Jews:
We even just had a thread on this board regarding when “Christmas eve” is.
I think it’s a bit messier than that. There’s a motu proprio of 1969 titled Universal Norms on the Liturgical Year and the New General Roman Calendar which says:
In a way, the second half of this sentence contradicts the first, but it seems that the canon law line of argument is that the liturgical day begins at midnight but you’re still allowed to fulfil your duty to attend mass the evening before because some special rule says so.
Yes, in the Episcopal/Anglican Church(es) that is the case. This came up as a question to my Bishop a few years ago, and the response was that the liturgy of the feast day could be held after sundown of the previous day. This was welcomed by priests and deacons who could face an Easter vigil on Holy Saturday, ending at 1:00am Easter Sunday, then have a sunrise Easter service a few hours later, followed by a later morning service.
While it’s theoretically tradition for an Easter Vigil service to run from sundown to midnight, and there’s enough optional material available to actually fill that span of time, in practice I’ve never seen a church that actually schedules that. It does usually start after sundown, but then takes maybe a couple of hours.