Numbers 28:25 is part of a section outlining specific offerings and observances for various festivals and holy days in the Israelite calendar. Let's break down the meaning of this specific verse:

"On the seventh day": This refers to the seventh day of the specific festival or period being discussed in the surrounding verses. Chapter 28 of Numbers details offerings for the daily sacrifices, Sabbaths, New Moons, the Passover/Unleavened Bread, and the Feast of Weeks (Pentecost). The specific festival being referenced is likely the Feast of Unleavened Bread which lasts for seven days.

"you shall have a holy convocation": A "holy convocation" (or "sacred assembly") means a gathering of the people called together for religious purposes. It implies a formal assembly for worship, prayer, and/or instruction. This suggests a special, set-apart gathering focused on God and religious observance.

"You shall do no regular work": This command prohibits engaging in routine labor or employment. It signifies that the day is to be devoted to God, and not to one's normal occupation. This allows for full participation in the holy convocation and reflection on the significance of the day.

In essence, the verse means: On the seventh day of the specific festival being described (likely Unleavened Bread), you must gather together in a formal, sacred assembly for religious purposes, and you must abstain from your everyday work.

Significance:

Setting Apart Time for God: The verse emphasizes the importance of dedicating specific times and days to God, separating them from the ordinary routine of life.
Communal Worship: The "holy convocation" highlights the value of gathering together as a community to worship and learn about God.
Rest and Reflection: Abstaining from work allows for physical rest and spiritual reflection, deepening one's understanding and connection to God.
Obedience and Reverence: Following these instructions demonstrates obedience to God's commands and reverence for His appointed times.