SharePoint email reminder for a task's Start Date should be sent for each item individually according to a number of days specified in a custom list field.
Virto Software Components Used
An energy company has required the ability to send calendar event reminders to users within a schedule specified for each event. The reminders for list task must be sent in 2 or 3 days before Start or Due Date.
Office 365 Alerts add-in allows users to send reminders, using different types of custom schedules. Among other features, Office 365 Alerts add-in has the option to send alerts Before or After X number of days (X value is specified for each item in a number field of the list).
As an example, you can add a number column "Days number for alert" to your events or tasks list (or any other list type).
Then create a new reminder with Virto SharePoint Alerts and Reminders add-in and customize an alert sending schedule.
In this reminder, the email message will be sent at 11:00 in the X number of days (X is the value from "Days number for alert" field) before the Start Date.
You can also customize the reminder message view and add any field values to this email.
You can learn more about other options to send an Office 365 alert and download and try its 30-days free trial.