How can I set an alarm to repeat every few days on iPhone?

Re: "Is there any way to set up an alarm so it repeats every x amounts of days? "

The only thing I can think of is that calendar alerts may be set up ... although may not work as a wake-up alarm

However, if you're not already using them, Apple support's online articles may be more helpful:

Eg: You can setup a sleep schedule in Heath, (Mon week?) then add a second schedule (Tues week?)!

(Apologies if you already know about these schedules, can not tell from your post.)

Set up a Sleep Focus schedule in Health on iPhone - Apple Support

Eg: says:

Set up your first recurring sleep schedule

When you set up a Sleep Focus, you can set your next bedtime and wake up time. This schedule repeats only once. To set up a recurring sleep schedule, do the following:

1. Go to the Health app on your iPhone.
2. Tap Browse at the bottom right, then tap Sleep.
3. Scroll down to Your Schedule, then tap Full Schedule & Options.
4. Tap Set Your First Schedule, then select your options:

• Set the days for your schedule: Tap a day at the top of the screen to add or remove it from your schedule.
  • The schedule applies only to days shown with solid-color circles.
  • Adjust your bedtime and wake up schedule: Drag to change your bedtime, to change your wake up time, or the semicircle between the icons to shift both times simultaneously.
• Set the alarm options: Turn Alarm on or off.
  • When Alarm is turned on, you can choose a sound, its volume, a vibration, and the snooze option.

5. Tap Add.

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Add, modify, or delete sleep schedules

After setting up your first sleep schedule, you can do the following:

1. Go to the Health app on your iPhone.
2. Tap Browse at the bottom right, then tap Sleep.
3. After setting up your first sleep schedule, you can do the following:

• • Set up additional schedules: For example, if you have a weekday schedule, you can add a weekend schedule. See Add or change a sleep schedule.
  • Change schedules: You can adjust the bedtimes, wake up times, and alarm options for an entire sleep schedule or for only your next schedule. See Add or change a sleep schedule.
  • Turn off alarms: You can turn off alarms for all sleep schedules, for a particular schedule, or for only your next scheduled wake up time. See Turn off alarms and delete sleep schedules in Health.
  • Remove a schedule: See Delete a sleep schedule and its alarm.

More info. if needed:

Change your wake up alarm in Clock on iPhone - Apple Support

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Re: "make Alarm dates as customisable as they’ve made it for Reminders".

Although it won't help immediately: You can send a "feature request" to Apple using their feedback form:

If others also want this feature, and let Apple know, we may see an update that includes more flexibility.

Product Feedback - Apple