The use of dative, instrumental, accusative, or prepositional with various units of time is something you have to memorize. По субботам on Saturdays, в субботу on Saturday. etc. etc. Don't try to generalize because there really are only a couple dozen items to remember. Or get a good Russian Reference Grammar (not a regular grammar book), such as the one by T. Wade or Pulkina. I recommend the British one, which I use all the time while Pulkins sits on the shelf gathering dust. The problem with every Russian technical book (even cookbooks) is the lack of an index. God only knows how they manage without indexes. I suppose all their databases are in ISAM format as well. ==:^0 (pulling my hair out)