Deuteronomy 16:3 says, 'Thou shalt eat no leavened bread with it; seven days shalt thou eat unleavened bread therewith...' while 16:8 says, 'Six days thou shalt eat unleavened bread: and on the seventh day shall be a solemn assembly...'

When read carefully, the context of 16:8 is that of observances. The only observance on the six days is to eat unleavened bread; the seventh day observance is a solemn assembly. It is a positive command. On the other hand, 16:3 is a negative command, essentially stating, 'Do not eat leavened bread these seven days, eat it unleavened.' This is an overarching prohibition that applies to the entire seven-day period, in addition to the specific ritual eating focused on during the six days.