diff --git a/packages/mermaid/src/config.type.ts b/packages/mermaid/src/config.type.ts
index df87e9c40..1163016aa 100644
--- a/packages/mermaid/src/config.type.ts
+++ b/packages/mermaid/src/config.type.ts
@@ -1067,7 +1067,7 @@ export interface GanttDiagramConfig extends BaseDiagramConfig {
    * On which day a week-based interval should start
    *
    */
-  weekday?: "monday" | "tuesday" | "wednesday" | "thursday" | "friday" | "saturday" | "sunday";
+  weekday?: string | "monday" | "tuesday" | "wednesday" | "thursday" | "friday" | "saturday" | "sunday";
 }
 /**
  * The object containing configurations specific for sequence diagrams