Great Day
/ˌɡreɪt ˈdeɪ/Chiefly Christian Church
noun
1With the. The Day of Judgement. More fully Great Day of Judgement, Great Day of the Lord, Great Day of Wrath, and variants.
2A very important feast day or (less commonly) day of fasting; specifically. Easter Sunday.
Origin
Late Middle English. From great + day, in sense 1 after post-classical Latin dies magnus (Vulgate).