Hadith: ‘Ukkāzh, Mijannah, and Dhū al-Majāz were markets in the pre-Islamic period of ignorance. They considered it a sin to trade during the seasons. So it was revealed: {There is no blame upon you for seeking bounty from your Lord.}

عن عبد الله بن عباس رضي الله عنهما قال: كانت عُكَاظُ، ومَجِنَّةُ، وذُو المجَازِ أسوَاقَاً في الجاهلية، فَتَأَثَّمُوا أنْ يَتَّجِرُوا في المواسم، فنزلت: {ليس عليكم جناح أن تبتغوا فضلاً من ربكم} "البقرة" (198) في مواسم الحج.
[صحيح] - [رواه البخاري]

‘Ukkāzh, Mijannah, and Dhū al-Majāz were markets in the pre-Islamic period of ignorance. They considered it a sin to trade during the seasons. So, {There is no blame upon you for seeking bounty from your Lord} [Al-Baqarah: 198] was revealed concerning the Hajj seasons.
[Authentic hadith] - [Narrated by Bukhari]

Explanation

These were marketplaces of the polytheists, where they traded during the days of Hajj. The Companions (may Allah be pleased with them) worried that they would incur a sin if they traded in such places during the days of Hajj. So, Allah revealed this verse to show them that trading in the Hajj season does not invalidate Hajj, especially when the rites are performed as prescribed. Trading during Hajj is allowed. However, it is better to devote oneself to performing the rites of Hajj.