呢个係咩文法？？

咩結構？
'planned' is a past participle that modifies 'anything',

it is in the past participle form as its meaning is in the passive: the action of 'plan' is done by the person.

it is easier to understand like this:
do you have anything [that is] planned for this Saturday?
幾時用？
when you want an adjective to modifies the noun:
e.g. Did you see anything broken?
Do not leave anything unfinished. I need you to have everything
done before you leave.

i hope that i have helped u.

pls do not delete this answer. thanks!

2011-06-29 11:02:03 補充：
the question is not in the passive voice.

do you have anything = simple present tense _ question
planned = here in this sentence functions as adjective to modify anything
for this Saturday = adverb of time

This is a form of passive voice.
The meaning of the sentence is: Do you have anything which is planned for this Saturday?