You are looking at the first warm weekend in April to get the block together, clean up the yards, and share some food. The real lever here is lead-time, because mulch, trash bags, and bulk food items always get picked over once the weather actually breaks. Give yourself a few weeks to lock down the date and figure out who is bringing what before people make other plans.
We are looking at about $250 total for supplies, food, and drinks if you are splitting the load with a few neighbors. Spreading that cost out needs a clear timeline so you aren't stuck paying for everything at once right when the hardware store lines get long.
That intro window is a hard deadline, not free money, and any leftover balance after those 18 months will start accruing interest.