Learn Russian: По воскресеньям мы катаем большой мяч по траве. - On Sundays we roll a large ball on the grass.

Катаем is the multidirectional verb. We use it here for two reasons: the action is a repeated habit (по воскресеньям), and the rolling is happening all around the grass (по траве) rather than in one single, specific direction.

Why do we use По воскресеньям instead of В воскресенье for On Sundays?

To express an action that regularly repeats on days of the week, Russian uses the preposition по followed by the Dative plural of the day. Here, воскресеньям is the Dative plural of воскресенье.

Why does the sentence use по траве for on the grass rather than на траве?

While на траве means statically resting on the grass, по + the Dative case (траве) implies motion along, across, or around a surface. With multidirectional verbs of motion like катаем, this combination perfectly describes moving all over an area.

Большой мяч is the direct object being rolled. Why didn't its ending change?

Because мяч is a masculine, inanimate noun. In Russian, the Accusative case (used for direct objects) looks identical to the Nominative case for inanimate masculine nouns. Consequently, the modifying adjective большой also keeps its Nominative-looking form.