、。
I'll keep Thursday open for you.
Literal
Thursday, you [for-のために] open-(in-advance)-[から].
あけとく is the colloquial contraction of あけておく — the te-form of あける ('to open / make empty') plus the auxiliary おく ('do in advance, leave in a state'). The contracted ておく → とく is everyday casual speech. Trailing から here is not 'because' but a sentence-ending particle expressing a friendly assurance — 'so [the matter is settled].' Whole register is intimate and casual.