Come whenever you like.

Itsudemo anata ga sukina toki ni kinasai. : Come whenever you like.
itsudemo : any time, whenever
anata : you
sukina : favorite
sukina toki : favorite time
toki : time
ni : at, in
kinasai : come (imperative)
Itsudemo anata ga sukina toki ni kinasai : anytime, you, like/favorite, time, at , come : Come whenever you like.

points;
...demo : any...

for example
nandemo : whatever
nan/nani : what, anything

daredemo : whoever
dare : who, anyone

dokodemo : wherever
doko : where, any place

Itsudemo (sentence(subject)+(predicate)) toki : the sentence modifies "toki" and "toki(time)" modifies "itsudemo"

Itsudemo (anata ga(you))+(sukina(like)) toki : any time , you like, time : any time you like.
If you put "ni (at)" at the end of above sentence,
Itsudemo (anata ga(you))+(sukina(like)) toki ni : means " at any time you like".

You can also use this "...demo" sentence like this;
Itsudemo daijoubu desu. : It's OK at anytime.
daijoubu desu : It's OK.

Daredemo daijoubu desu. : Anyone is OK.

Dokodemo daijoubu desu : Any place is OK.