Does Psychosis Crawler die in the middle of Days Undoing?
Psychosis Crawler won't die during the resolution of Day's Undoing.  Toughness check is done as a state-based action.  State-based actions are checked after each spell/trigger is done resolving.

704.3. Whenever a player would get priority (see rule 116, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any ...
704.4. Unlike triggered abilities, state-based actions pay no attention to what happens during the resolution of a spell or ability.