I prefer the end of the month.
Then people have time to purchase donation items, and there is the whole air of suspense that makes seeing what the new item is worth it. n_n
Besides, it makes more sense to release a May donation item at the end of May, because then the motivation can't soley be based upon the pretty donation letters.
Besides, if items were released at the beginning of the month there would be no Sealed Letters. It would eliminate them.
The middle of the month seems more sensible than the beginning, but I still don't like it. It still kills some of the suspense and doesn't make as much sense as the end.
A donation item should be released at the begining of the final seven days of the week, so that there is a full week to donate and receive a letter when you know what it contains, but a decent time to make a donation that comes from the heart.