It’s part of Holy Week, the week before Easter
Maundy Thursday commemorates the Last Supper and Jesus getting arrested. At the end of the service at home they always do this thing at the end where they take away all the fancy vestments and candles and stuff and dim the lights
Then when you go to the Good Friday service (commemorating his crucifixion) the sanctuary is all bare
Then you go on Easter Sunday and tadaa, everything’s all bright and fancy again