Referring to the current day.

आज

English Usage: Today is a beautiful day.

Hindi Usage: आज एक खूबसूरत दिन है।

A period of seven days.

सप्ताह

English Usage: There are seven days in a week.

Hindi Usage: एक सप्ताह में सात दिन होते हैं।

A specific week referred to, often in a context of scheduling.

विशिष्ट सप्ताह

English Usage: The meeting is scheduled for next week.

Hindi Usage: बैठक अगले सप्ताह के लिए निर्धारित है।