Use the adjective invariable to describe something that's unlikely to change, such as your invariable custom of eating deep-dish pizza every Saturday.
You can hear the word vary in invariable. Something that varies changes or is made up of different kinds of things. When you add the prefix in-, meaning "not," and the adjective suffix -able, you get an adjective to describe something that's not able to vary or change. Synonyms for invariable include constant, fixed, and immutable. Someone who's invariable in her habits may seem predictable and boring on the one hand, and steady and reliable on the other.