Begin with the season, then select the best day and time to hold your sale.
The season should be appropriate for the type of merchandise you want to sell. Snow-blowing equipment, for example, will sell better in December than it will in July.
The day of the week and time should be convenient for the customers you're hoping to attract. For example, few contractors will leave a job site in the middle of the day during their workweek to attend a liquidation sale. But if you schedule it for a Saturday morning, they'll be there. Similarly, hair salons and restaurants are typically open on Saturdays, but closed on Mondays. Make it easy for the owners of these businesses to attend your sale. Schedule it for a Monday.
Of course, you also need to take your marketing plans into account. Find out when the trade journals you want to advertise in are published and how much lead time they'll need to include your ad.
