diff --git a/x-pack/plugins/alerting/server/lib/is_rule_snoozed.ts b/x-pack/plugins/alerting/server/lib/is_rule_snoozed.ts
index 927a21cd8cb14..a950171beb291 100644
--- a/x-pack/plugins/alerting/server/lib/is_rule_snoozed.ts
+++ b/x-pack/plugins/alerting/server/lib/is_rule_snoozed.ts
@@ -10,6 +10,8 @@ import { SanitizedRule, RuleTypeParams } from '../../common/rule';
 
 type RuleSnoozeProps = Pick<SanitizedRule<RuleTypeParams>, 'muteAll' | 'snoozeSchedule'>;
 
+const WEEKDAY_CODES = ['SU', 'MO', 'TU', 'WE', 'TH', 'FR', 'SA'];
+
 export function getRuleSnoozeEndTime(rule: RuleSnoozeProps): Date | null {
   if (rule.snoozeSchedule == null) {
     return null;
@@ -59,7 +61,8 @@ export function isRuleSnoozed(rule: RuleSnoozeProps) {
 function parseByWeekday(byweekday: Array<string | number>): ByWeekday[] {
   return byweekday.map((w) => {
     if (typeof w === 'number') return w;
-    if (['SU', 'MO', 'TU', 'WE', 'TH', 'FR', 'SA'].includes(w)) return w as WeekdayStr;
+    if (WEEKDAY_CODES.includes(w)) return w as WeekdayStr;
+    // Handle nth day codes, for example, +1MO means the first Monday of the month, -1FR means the last Friday, -2TH means second to last Thursday
     const dayOfWeek = w.slice(-2) as WeekdayStr;
     const nthDay = w.slice(0, 2);
     return RRule[dayOfWeek].nth(Number(nthDay));