This year, Corpus Christi unusually falls in May. Thursday, 22 May. Usually it is in June and makes for a first long weekend after the close of school. With it falling in May, and on right after the election long weekend, expect lots of commercial activity. The stores already had their long weekend, so I would say it will be a good enough time to go shopping, at least at the larger stores. In the tourist destinations, the larger stores remain open on holidays because they do their best business on holidays, when droves of Dominicans head to the resorts.
To check for other Dominican holidays, see
http://dr1.com/travel/prepare/holidays.shtml