This is the 276th most frequent Lithuanian word.
sekmadienį
"On Sunday."
In this sentence, 'sekmadienį' specifies the time of habitual action, also using the accusative case to denote the specific time.
Visada sekmadienį ruošiamės naujai savaitei.
We always prepare for the new week on Sunday.
The word 'Sekmadienį' here indicates the day of the week in the accusative case, showing when the action would take place.
Sekmadienį mes eitume į parką.
On Sunday, we would go to the park.
Here, 'sekmadienį' is modified by 'praėjusį', showing a specific Sunday in the past, still using the accusative case for designation of time.