Date and Time: Search on-line, and you'll find a lot of commentary about the appropriate date and time for parties. As usual, everyone has an opinion. Mine: do what works best for you and your family (within reason), as that is likely to give you the best results. For me, Saturday parties from about 11am-2pm or 3pm-6pm (so as to avoid naps) and Sunday parties starting after about 12 or 1pm (so as to allow churchgoing families time to attend services in the morning) work well. But you know yourself and your party-goers best, so schedule accordingly. Within the last few months, I went to a party for a good friend's daughter that was scheduled from about 6-8pm on a Sunday night at a bounce house place -- it was well attended and a lot of fun!
Location: I love to throw parties at our house. I know this is not an option for everyone, but sometimes it may be more of a possibility than you think -- if you get creative. Our home is a little over 1200 square feet, but we've thrown parties with nearly 100 guests in attendance. We have a pretty good flow in our house and use multiple rooms, along with the front and back yards and the garage. It works for us and allows me ample time to set up decor, etc., the day before the party (as opposed to an hour or two before, which is sometimes all you may have at a venue).
If you don't think you can make your house (or the house of a loving family member or friend) work, then start researching venues about 3 months prior to the party and book as soon as possible. Less time may be necessary, but you definitely don't want to miss out on reserving the perfect spot by only a few days! My sister-in-law recently reserved an awesome city park with water features for the kiddos to play in for my niece's upcoming Bubble Guppies party. The park fits the theme perfectly, and she's able to get there and decorate about 3 hours before. Making this happen meant researching the park's reservation policies and calling in at 7am on the first day she could make a reservation for her date, but her research, planning, and diligence paid off, and she got the date and time she wanted.
A Final Note: Time2planaparty is about making the "time" to plan a party. If you're reading this with a week to go before your special event (as opposed to three months), don't despair -- just breathe and do your best. The ones you love will love you for whatever you're able to put together, and that's the important thing. But next time, consider starting the planning about three months in advance and do little bits at a time. Having that "time" makes planning more fun, less stressful, and gives you a chance to enjoy the creative parts!