This is the set time of year that they celebrate the Passover. The full meaning of this statement can be made clear. Alternate translation: "On the fourteenth day ... and observe it, for this is the time you do celebrate it every year" (See: [[rc://en/ta/man/translate/figs-explicit]])
# 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. (See: [[rc://en/ta/man/translate/figs-parallelism]])
Here the phrase "keep it" is an idiom which means to observe it. Alternate translation: "You must observe it" or "You must celebrate it" (See: [[rc://en/ta/man/translate/figs-idiom]])