I'd just like to add that you give the party when you feel it's best for you. Others can work around it, or decline to come. You can't make endless adjustments to accommodate everyone. Some people go to church on Sunday, some go on Saturday, some don't go at all or don't go every week. Pick the time that is best for your family, your child, and the close friends you really want in attendance.
Keep the group manageable in size. Don't invite the whole class just because your child is in public school. I don't think you want to be going to 22 parties this year, and neither does anyone else. Just invite - through private invitation, not a teacher-distributed flyer! - those few kids your child is friendly with. Teachers DO NOT need one more thing to do, and we've seen many posts on Mamapedia of parents who DID send an invite home via school backpack and then never got RSVPs. They'll never know if the parents are rude, or if they sent an RSVP back via the "backpack system", or if they never got the flyer because the kid lost it or it's buried in the bottom of the backpack covered in apple juice.
Don't tell people that gifts aren't necessary or that they don't need to spend much. You can't tell people what to give. Don't say anything about it at all. People won't come without a gift anyway, and some even feel manipulated by you telling them not to bring your child a present! The reason birthday parties become so expensive is that people go overboard in the entertainment and goody bag department, and that people invite too many people and then too many guests have to attend too many parties. We all need to keep the parties small enough that close friends can afford to attend, so that we can afford to feed and entertain those who come, and so that our kids aren't receiving an overload of gifts to make it a present-festival instead of a celebration. Guests (especially kids) don't want to sit around and watch a child open 25 gifts, and no kid wants to spend the next month writing 25 thank you notes. Over-indulgence actually ruins the festivities.
Your invitation should make it clear what the activities are. People in your area know about the weather. Tell them to bring boots for water play. You can do a few simple games if you want - or have them available in case there's a need. The kids in our neighborhood are going back to easy things like potato races (you need a half dozen potatoes and some wooden spoons from the dollar store) - if you don't get to the game, no problem. Eat the potatoes and save the spoons for next year. They do 3-legged races - you need enough lengths of rope or thick elastic to create some bindings. They do sack races - you need a few old pillowcases that you're using for laundry bags or are ready to throw out. Simple simple simple. Kids have more fun, parents enjoy the party instead of turning parties into budget-busters and supervisory nightmares, and everyone just gets back to basics, like celebrating the birthday child and having some cake.