Depends what you mean by a day pack.
By the time you put together a day pack with all the stuff you might need during the day, there isn't really much left to send ahead
one spare set of clothes and some soap, toothpaste, a toothbrush............
Some day packs are quite well made, with good padding, hip belt and support. Some are very flimsy and will be a real pain to carry............
IMHO it would be better to have a proper small pack rather than an ultra lite day pack if you are thinking of that, as it will also be fairly light, but more importantly,
comfortable. .
Then just pack light.....
I carry a 34L pack that is really comfortable. I would prefer to use that; and have; rather than an ultra lite day pack regardless of how little I was packing. My wife carries a 24L pack. Small, light, very comfortable. Fits all her gear in it.
For example.
My wife's pack. Technically....A Day Pack. Because it's small I guess.
https://www.paddypallin.com.au/osprey-sirrus-24-day-pack-womens-updated.html
My pack: Also referred to as a Day Pack:
https://www.paddypallin.com.au/osprey-stratos-34-day-pack-updated-39080.html
In these we can carry gear for 40 days.
For 5 days....................I would carry the exact same gear. Nothing different. 2 days or 50 days, it's basically the same gear for me.
It's about travelling light! My total gear is 5.5 kg + food and water. Of that, probably only 1.2 kg is not required during the day. Spare clothes etc.
This is an Ultra Light day pack. I take one and carry it for going shopping.
If I had to send gear ahead due to injury or whatever, I would use this to send the gear, and still use my back pack for walking. It's more comfortable.
The ultra lite day pack would be far too uncomfortable to carry over any distance on my back, like more than a couple of kms, even with just a couple of KGs in it. Small straps that will dig into shoulders. no support etc.
It's really a shopping bag with shoulder straps even though it looks good in the pics....
https://www.paddypallin.com.au/sea-to-summit-ultra-sil-daypack.html
Happy planning......