What day of the week was the day of Pentecost falling on in Acts 2? Was it Saturday or Sunday? Or some other day?
There is not full agreement among different people. According to the People's New Testament it fell on a Sunday (50 days after Christ rose from the dead) and this seems to make sense to me because the death, burial, resurrection, and blessing of the Sprit all have a meaningful relationship with existing Jewish feats.
I agree with the People's New Testament calculation partly because it matches so well with the symbolism of the events, which seems to be a biblical way of events naturally occurring. Sometimes when we get deep into the nifty gritty about Jewish things, such as this, I prefer to find a source that Jewish Christians support as they are more aware of the Laws of Moses then regular Christians are (although not always of course).
From a Jewish Christian site I found an excellent article on the subject explaining why it fell on a Sunday and why objections to this argument are not well founded:
The answer to this question is found in the Hebrew Bible--the day was Sunday. In order to understand the parallel, one must assume and then accept the Scriptures with the PLAIN and NORMAL understanding of its own words and meanings.
In other words, the VERY SAME DAY that the Mosaic Covenant was inaugurated to the nation of Israel on Sinai (Sunday) was the VERY SAME DAY that the New Covenant was announced to the nation of Israel (Sunday). The timing was exactly 50 days from the day as mentioned in Leviticus 23:16.
Thus in the former case, the law of God was written on stones; in the latter, the law of God was then written on hearts. In the former case the Feast of First Fruits marked the blessing of water from heaven (that precipitated abundant crops); in the latter case, this water from heaven was eternal life (that precipitated the "first fruits of the Spirit" mentioned in Romans 8:23). Please click here for an overview depiction to compare and contrast the concepts and the respective timelines concerned.
The parallels are remarkable (and breathtaking) when we assume and then accept the Scriptures with the PLAIN and NORMAL understanding of its own words and meanings.
In summary, there is no academic sophistry here--"Pentecost" thus occurred on a Sunday as we view the parallels from the Hebrew Bible.