Customizing date headers with the Display preferences
The Display preferences are the second pane of the preferences window, available from the OmniPlan menu.
You can customize the format of dates and date ranges that appear at the top of OmniPlan's various charts.
Here you can choose which day OmniPlan should consider as the first day of the week. 
The pop-up menu determines which chart header scale you are editing. The different time scales appear in the headers of the Gantt chart, resource timeline, and work week chart, depending on the zoom scale. 
Click the plus button here to add a summary header to the current scale; this makes an extra layer of headers above the normal one in the Gantt chart and the resource timeline. This extra layer gives you more room to show, for instance, the week above the day, or the month above the week. The summary header format can be customized in the same way as the regular date header format, by dragging tokens from the box below and by typing directly into the field. Here we have a summary format that displays the word "Week" followed by the number of the week. To get rid of the summary header, just click the minus button to the right of the field. 
Here's where you can assemble a date format. Drag tokens from the box below, and type spaces or characters wherever you want them to appear. Some tokens offer information about the date at the beginning or end of a week or quarter. This is so that you can, for example, create a week format which indicates the first and last days of the week. Here we have a format that displays the short name of the month, followed by the date. 
Tokens can be dragged to the format field above to construct a date format. Click the triangle on the right side of a token to change the display of the token. For example, this token represents the month; you can choose to display the month as a full word (January), an abbreviation (Jan), a two-digit number (01), or a one-digit number (1).