There is no specific date. Any time you return to a dungeon AFTER defeating the storyline boss there, it will be a reset dungeon with the optional boss sitting in the room where the boss was. After defeating it, don't forget to check the room for a rare treasure it drops.

The bonus enemy dungeon will appear When people you rescued JOINED you For example:
you have save yukiko from her dungeon BUT she needs rest After she done resting she will talk to your group right ? then Yukiko joined your group from that day onwards Bonus boss on yukiko castle is available. its the same for the others you will rescued,The bonus boss is available When the people you rescued joined your group**SPOILERS**ALERT**START**HERE** except for nanako her Dungeon bonus boss will available when she become worsened from her sickness (adachi calls that Nanako health suddenly got worse)**SPOILERS**ALERT**END**HERE**ANOTHER**BIG**SPOILERS** START**HERE**Another exception in Magatsu Inaba and Yomotsu Hirasaka They'll be no bonus boss since there are 2 boss in Magatsu inaba And you'll be leaving after Yomotsu Hirasaka They're wont be any Special drop too**SPOILER**CHAIN**COMBO**ENDS**HERE**

For detail see the FAQs

*SPOILERS* one more i dont know the exact dates and sorry i dont know when Void game quest bonus boss will appear*Spoilers*

And why the people must in your group?
for example when you buy weapons in daidara yukiko weapons(fans) is not shown until she in you group right
that the same thing with the bonus bosses since they drop special weapons to the people you rescued Except for *Spoilers* Void game quest and Heaven*Spoilers*

*SPOILERS* sorry i dont know the right times when bonus boss appear, but you can check it by going inside tv several days after you defeat main boss, rise/teddy will tell there is something wrong inside the TV

I don't know the exact days, but basically a dungeon will have its bonus boss available when the NEXT dungeon first becomes available. So the Yukiko's Castle boss won't appear 'til the Bathhouse is available, and so on.

Note that Magatsu Inaba and (obviously) Yomotsu Hirasaka don't have a bonus boss.