Can I add a plan before I know the exact day?

Yes. Each month has a Sometime strip for plans that belong to that month but have no day yet. Point at a month and click the lightbulb by its name, or click the + at the end of a strip that already has items. You can also switch Date to Sometime in an event's panel, for a new event or one you already have.

If you don't see the lightbulb: with a mouse, it only appears while the pointer is over that month. Or create an event as usual and switch its Date to Sometime. On a calendar you can't edit, such as one shared with you to view only, nothing can be added, so the lightbulb for adding doesn't appear.

When you know the day, drag the item onto it and it becomes an ordinary event. On a touch screen, open the item and give it a day instead.

Sometime works on every plan. On Free, each item counts as one of your 30 events. Sometime items appear in your calendar and in calendars you share, but not on published pages, in CSV export, or when the calendar is overlaid on another one.