Learn Croatian: Jedem juhu svaki dan.

Juha is a feminine noun ending in -a. Because it is the direct object of the verb jedem (the thing being eaten), it takes the accusative case. For feminine nouns ending in -a, the accusative ending is -u, making it juhu.

Where is the word for "I" in this sentence?

In Croatian, the pronoun ja (I) is usually dropped because the verb ending already tells you who is doing the action. The -em ending on jedem clearly means "I eat." You would only add ja for emphasis.

Why don't we change the ending of svaki dan if time phrases also use the accusative case?

Svaki (every) and dan (day) are masculine words that do not represent living things (they are inanimate). For masculine inanimate nouns and their adjectives, the accusative form is exactly the same as the dictionary form, so they look like they haven't changed.

What is the dictionary form (infinitive) of the verb jedem?

The infinitive is jesti (to eat). It is an irregular verb, which is why the stem changes slightly from jes- to jed- when you conjugate it in the present tense.