It was traditionally on the 7th day which is 8/21 sunset to 8/22 sundown.
I'm a bit iffy on sunset/sundown. It seems to me that "full dark" is sundown and owing to the time change each week that makes it slightly over 24hrs otherwise it isn't quite 24hrs.
The sunday thing is up for debate but nowhere that I can see was the sabbath "re-set" up as being on the 8th day (or day 1?)
Take it before the Lord. A day of rest and not pleasure seeking dedicating it to holiness in a concentrated fashion vs passively each day, may be something for you to consider. I don't want to get much into that because it's a pretty personal thing I feel.
There's a lot more to it and I've already posted about it so I'm not going to get into it.
Either way we aren't to judge people with respect to the Sabbath. Colossians 2:16. Also Mark 2:27 is important to note the meaning behind it. It's made for us not us for it.
Also Luke 6:5 and the preceding verses could tie into Mark 2:27 as well as healing on the Sabbath or if a situation arises Luke 14:5 (like getting called into work or an emergency or perhaps going to catch your dog who escaped the fence, etc.)
We aren't under the law as far as Sabbath breaking is concerned, however engaging the heart of why the law was given (and the lawgiver) is of import depending on personal burdens/relationship