。
She's throwing a party tonight.
Literal
She [topic-は] tonight party [object-を] holds.
開く ('to open') is the standard verb for hosting events — parties, conferences, exhibitions, even meetings. The metaphor is of 'opening up a space' for an event, not of physically opening anything. Other natural collocations: 会議を開く ('hold a meeting'), 展覧会を開く ('hold an exhibition'). For larger or more formal gatherings, you'll also hear 催す or 開催する.