Using ceiling hook with no joist or anchor/toggle? Is it ok to do for string lights? What hook to use?
If you have a plaster ceiling and it's in good shape, not coming away from the lath anywhere, there are some screw types and techniques that could reasonably hold weight like this.  I don't recommend it but it's possible and you need to experiment to find what works well in your plaster. It varies.  If you do this, also follow the advice below for the FIRST HOOK in drywall.
Assuming however you have a drywall ceiling:
The FIRST HOOK, the one that has the long tail of the power cord, switch, and plug dangling from it, should be a strong hook screwed deep into a joist.  At least a #8x2&quot; eye hook.  This will take the strain of being wiggled and pulled when the plug or switch is handled by human hands.  The cable should be firmly attached (eg tight wire ties) to this hook in order to prevent further downstream hooks from taking any stress from handling.
After that for the subsequent hooks: Options to hang lightweight things on ceilings and leave no, or minimal damage after removal:

Removable sticky-backed  stretch-to-remove such as 3M command strips. Those should hold the weight of this string and can be removed with no trace but sometimes the paint comes off with them.
Use steel screw hooks or screw eyes (not cup hooks), find the thinnest longest one you can.  I've seen #8x1.5&quot; and M3x1.5&quot;.  If you could find a #6x1.5&quot; that would be great but I can't see one. Screw through the drywall into a joist without drilling.   The purpose of the joist is not because you have a heavy piece to hang.  It is to allow you to use a very thin screw that will hold well, and can be removed with minimal damage to the ceiling and no drilling.  The hole left behind will be almost invisible without painting, with just some filler.
If you don't want to find and use joists then find the smallest plastic anchors you can, and use them with small cup hooks to conceal them.  Don't use decorative cup hooks without anchors.  They are too short to reach joists and won't hold without anchors.
If you just don't want to use a drill, buy self-drilling drywall anchors.  I don't recommend this, however, because if you do hit a joist with one of these you end up with a mess.  With very small plastic anchors (method #3) you can drill into the joist to accommodate them or cut them short at the surface.