Rosh Hashana 4a & Rosh Hashana 4b mentions a whole list of items that one does the sin of Baal T'acher. There is a dispute as to when this prohibition takes effect, ranging from the first festival after the vow, to Succos which followed a Pesach and Shavuos first. Is this sin applicable today if one makes a pledge to Tzedaka and did not fulfill it within the prescribed time? (sources)
Aruch Hashulchan Yoreh Deah 257 says that according to the Rambam and the Ran one would have done the Aveira of Baal T'acher by Tzedaka immediately if they do not give the Tzedaka right away.