There's no single best timing for all businesses. The best starting point is to understand your audience and form a good idea about their behaviors.
The customers of a clothing company and those of financial services will not check their emails at the same time of the day. Meanwhile, stay-at-home moms and "digital nomads" can be more flexible and less predictable than typical employees.
To gather data about your customers, you can use Facebook Insights or Google Analytics. These tools will reveal their interests, locations, and demographics. You can make informed assumptions about their time preferences based on their lifestyle from this point.
Buyer Personas are data-based representations of your customers. By creating them, you'll gain a more personal and authentic appeal to your customers, who can turn into advocates for your cause and products by addressing your customer needs.
A business will usually have several buyer personas with unique specifications. Segment your customers based on their activities or how they use your product.
Try to be as inclusive as possible and specify:
The more detailed your buyer personas, the better. Buyer personas should include: