In days of yore, the 'send off' parties for the bride and groom-to-be were done the night before the wedding. We can think of no more perilous plan. The image of a red-eyed groom or the appearance of a bride wearing an ankle cast from her dancing-on-the-table' injury bodes badly for the future of the happy couple. There are several popular time-frames that leave lots of time for merriment without the associated visual reminders of the party itself. Here are a few.
If you happen to be lucky enough that all the members of the wedding party live in fairly close proximity, you have more Bachelor or Bachelorette party-planning flexibility than those whose best man lives across the continent. It is probably unreasonable to expect any member of the party to travel long distances twice, just as it is unreasonable to have the party without all the important players. If you can plan the party a month ahead of the wedding itself, it gives you the invitees the chance to get to know each other, if they don't already, and so, by the big day they'll be old friends.
Planning the Bachelor or Bachelorette party just two weeks before the wedding can serve to take the edge off jitters both the bride and the groom may be experiencing. It's a great way to soothe them, on the one hand, and excite them on the other. Being with treasured friends and people who will play a part in the big day can be very helpful to bring the couple into the realm of reality. (Some may be trying to forget that the wedding is merely 14 days away, and others are certain the big day will NEVER ever arrive.)
There is a certain tidiness to planning both the Bachelor and Bachelorette parties on the same day at different venues, or different areas of the same hotel, for example. This way, neither the bride nor the groom is left to brood alone while the other has a big celebration. It is also handy for the bridal party members who are traveling back and forth for the event. As frequently happens, members of the bridal party are related, are old school chums, live near one-another, or are traveling from the same location. This makes carpooling possible and creates another bonding experience if time has passed since these folks were together.
Of course, you may also have a co-ed, or Jack and Jill Bachelor/Bachelorette party using any of these schedules. The point, of course, is to have fun - so don't stress over it.