Actually that should be an element, not modifier class

app/assets/stylesheets/events.css

@@ -8,7 +8,7 @@
   padding-block-start: var(--block-space);
 }
 
-.event--wrapper {
+.event__wrapper {
   align-content: end;
   display: grid;
   gap: var(--grid-lines);

app/helpers/events_helper.rb

@@ -25,7 +25,7 @@ module EventsHelper
 
   def event_cluster_tag(hour, col, &)
     row = 25 - hour
-    tag.div class: "event--wrapper", style: "grid-area: #{row}/#{col}", &
+    tag.div class: "event__wrapper", style: "grid-area: #{row}/#{col}", &
   end
 
   def event_next_page_link(next_day)

test/controllers/events_controller_test.rb

@@ -8,7 +8,7 @@ class EventsControllerTest < ActionDispatch::IntegrationTest
   test "index" do
     get events_url
 
-    assert_select "div.event--wrapper[style='grid-area: 9/1']" do
+    assert_select "div.event__wrapper[style='grid-area: 9/1']" do
       assert_select "strong", text: "Layout is broken"
     end
   end
@@ -18,7 +18,7 @@ class EventsControllerTest < ActionDispatch::IntegrationTest
 
     get events_url
 
-    assert_select "div.event--wrapper[style='grid-area: 14/1']" do
+    assert_select "div.event__wrapper[style='grid-area: 14/1']" do
       assert_select "strong", text: "Layout is broken"
     end
   end