Personally, I wouldn't ask anything but some variation of "when are you free?" over text. If you ask some specific day and she's busy then it starts to become way too much boring texting and hard to tell if she interested. I would hope you're dating high quality women with lives and goals. They may be legitimately busy with things on a specific day. If they're interest they'll definitely be able to find a day to spend with you soon, but don't expect to be number one on their list right away.
If you want to schedule a specific day, call her and state why, such as hey "Friday is going to be a nice day, finally no rain, lets go have some fun outdoors". Odds are if she answered the phone in the first place, or called you back, that she is interested so if she is busy on that specific day just fall back and ask what evenings she has free.
