There were several dates when specific families donated wood to the Temple and observed that day as a private holiday. One of those days was the 20th of Av. (Taanis 26a) It was possible that these days would coincide with Shabbos. When that occurred, the family would postpone the donation to Sunday, and not advance the observance to Friday. (Megillah 5a)
In two locations, Rashi seems to offer divergent explanations for this practice. In his commentary to tractate Megillah, he explains that the family could not advance their offering to Friday because the time-frame of their obligation had not yet arrived. When it did arrive, on Shabbos, they could then wait until the first opportunity to fulfill their obligation, on Sunday.
In his commentary to the Rif, however, he explains this differently. The previous family’s obligation extends until the onset of the next family’s obligation. The family whose obligation fell on Shabbos cannot advance their offering to Friday, because they would be interfering with the previous family’s time-frame.
