en_tn/num/09/03.md

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

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)

the fourteenth day

"day 14" (See: rc://en/ta/man/translate/translate-ordinal)

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)

You must keep it

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)