"I have plans for Saturday."
Translation:Tengo planes para el sábado.
Can't explain for all other cases (I sometimes get things wrong because of articles!) but for the days of the week, I've always understood it to mean it's a particular day you're talking about, (kind of like in English we would say I have plans for the 15th), so use the el, even if you wouldn't say the in English. And if you mean, for instance, all Saturdays in general, you would say los sabados.
i really struggled with this to start with... my approach is to use flash cards. Every time you encounter a new irregular verb, use something like spanishdict.com to write down all the conjugations on a small card, and keep the card handy. You'll build up quite a pack of cards, you can shuffle these and look at different cards each day to jolt your memory.
It depends on if you want to say ON Saturday ( EL sabado) el and los=on
I work ON Saturday. Trabajo EL sabado.
I work every Saturday. ( I work ON all the Saturdays) Trabajo todos LOS sabados.
I have plans FOR Saturday Tengo planes para sabado.
What day is it today? It's Saturday. Qué día es hoy? Es sabado.