It it wise or even possible to not turn in completed field research quests in order to turn them in on another day? For example I'm in a highly active area on a Saturday and usually busy during the week. I would then complete one quest on Saturday and keep three quests completed without turning them in, giving me the possibility to turn them in on Sunday, Monday and Tuesday. Would it count as 4 different days or is there some magic that resets the quests? (Ongoing quests are not reset, so I doubt that would happen.)
Yes, it is possible to not turn in completed field research and obtain the stamp on different day.
I tested this myself just a few days ago, knowing I would have not had the chance to complete a field request the next day. I had already received a stamp for that day, so I completed a field research but did not turn it in. The following day, I was able to redeem my reward and received the stamp.
This is also confirmed on PokemonGoHub
You can delay picking up rewards to get a future day stamp after midnight
You can keep the Field research when completed turning them in when needed for stamps. Completed research fills the page and you can’t get more tasks until you’ve turned in the fulfilled ones. The special research self completes when fulfilled.