Catholics are explicitly, overtly allowed to work on Sunday. It is, however, a venal sin to work on Sunday (a) at the expense of worship and reverence (in other words, don't skip Mass) and (b) when you had a reasonable option not to. Five generations of American Catholic men appear to have established the convention that Sunday afternoon football incurs minimal expense to revering Jesus.
Most of the biblical emphasis a Sunday sabbath comes from the fact that Easter was celebrated on Sunday, and a few verses saying that religious holiday observance didn't matter.Some Catholic Catechisms mentions that there is no biblical support for a sabbath on Sunday, and justify it by saying that the church has the authority to change religious holidays. I'm not sure if this is atypical compared to most catechisms, I am not very familiar with Catholic theology.
Explanation for change:http://www.religioustolerance.org/sabbath.htm
Catholic Catechism excerpts: http://www.biblestudy.org/godsrest/catholic-protestant-confe... The site is definitely biased, but it has a nice collection of excepts from christian and catholic doctrines.
Second, Going to church on Sunday and believing Sunday to be a Sabbath day are two totally different things.
In fact, keeping Sabbaths is not a Christian thing to do at all. Christians do it, but that's because they think they are (really, really want to be) Jews. Tons of the crazy stuff Christians do are because they try to adopt little pieces of the Jewish economy (10 commandments, feast days, sabbaths, etc, etc,).
I don't know when/why Sunday became the de facto day for Christian church-going, but I can answer for myself, that I attend church on Sundays because it is the most convenient day (where I live) to do so. I don't regard it higher than any other day of the week (ok, maybe Monday ;).