# On the fourteenth day ... at its fixed time of year

"On the fourteenth day ... and observe it, for this is the time you do celebrate it every year" 

# the fourteenth day

"day 14" 

# follow all the regulations, and obey all the decrees

These two phrases mean basically the same thing and are combined to emphasize that they needed to obey the commands.

# You must keep it

"You must observe it" or "You must celebrate it"