One of the most popular Memorial Day activities is the barbecue. Most people invite a lot of friends and family to join in the fun, which makes it a great place to do some networking. There's a good chance a lot of the people you see haven't seen you in a while, so they won't necessarily know you are currently looking for a job
The danger here is you don't want to come off as begging or desperate. You also don't want to ruin the festive atmosphere by bringing up work. That's why it's important to bring up your job search in a casual manner. Here's an example of what you could say in response to a question such as "What have you been up to lately?"
"Well I've been pretty busy these days. I've been spending a lot of time looking for work which, as you probably know is pretty frustrating. I've gotten some leads but it's still a struggle."Something like this will hopefully lead the person to offer some assistance, assuming they have contacts in the nonprofit sector. If you don't get that kind of response, you can always ask if they could offer any assistance. But that should wait until the end of the conversation.
Make sure you have a good Memorial Day weekend regardless of what happens with your job search. Consider these three days as a reward for all of your hard work so far!