What decides whether you use use the locative case when it comes to times and dates? For example, we have the sentence here: "bu Ağustosta o yüzüyor", and we also have another example where it is not used "bu ilkbahar Yunanistan'a gidiyoruz". What difference in meaning does it make when you use the locative?
in general when you use adjectives like 'bu' (this) and 'geçen' (last) you don't have to use -de/-da. bu ağustos o yüzüyor is also okay. if you want to use -de/-da with these adjectives you should make sure that these time words can be used with -de/-da alone. for instance you should be able to say 'ağustosta' in order to say 'bu ağustosta'. you can't say 'yazda', you can say 'yazın' so you are not able to say 'bu yazda'. and again you can't say 'bu çarşambada' because of the same reason.
so why you want to use -de/-da with bu and geçen? well in written language people prefer to use them if it's possible because it sounds more clear and formal. so in written language people would also choose to say 'bu ilkbaharda' because you can say 'ilkbaharda' (in spring) alone. also, when speaking people are more likely to add -de/-da when that time span can be rarely encountered in order to sound more clear for example months or special days such as christmas and new year.