大斋首日（又名圣灰星期三、圣灰日、灰日，英语：Ash Wednesday），是基督宗教的教会年历节期大斋期（四旬期）之始。
当天教会会举行涂灰礼，要把去年棕枝主日祝圣过的棕枝烧成灰，在礼仪中涂在教友的额头上，作为悔改的象征。
大斋首日是根据复活节的日期前四十天（不计算主日），因此每年的大斋首日的日期都不同。最早可以是二月四日，最迟是三月十日。
Ash Wednesday, in the calendar of Western Christianity, is the first day of Lent and occurs 46 days before Easter. It is a moveable fast, falling on a different date each year because it is dependent on the date of Easter. It can occur as early as February 4 or as late as March 10.
According to the canonical gospels of Matthew, Mark and Luke; Jesus spent 40 days fasting in the desert before the beginning of his public ministry, during which he endured temptation by Satan.[2][3] Ash Wednesday marks the beginning of this 40-day liturgical period of prayer and fasting.
Ash Wednesday derives its name from the practice of placing ashes on the foreheads of adherents as a sign of mourning and repentance to God. The ashes used are typically gathered after the palms from the previous year's Palm Sunday are burned.[4]
This practice is common in much of Christendom, being celebrated by Catholics, Anglicans, Lutherans, Methodists, Presbyterians, and some Baptist denominations.[
