The knotted cord
Take a length of plain cord and tie three knots in it, naming a wind you do not want with each knot. Hang it somewhere sheltered until your date passes, then untie the knots in reverse order and let the cord go.
This is the oldest weather charm in northern Europe, borrowed from the wind-sellers who once supplied sailors.
The sun candle
From the new moon before your date, light a yellow or gold candle for a few minutes each evening, facing west. Say the date and the place out loud. Snuff it, never blow it.
The value is the repetition. Twenty-eight evenings of naming the same wish is a remarkably steadying thing to do while you plan an event.
The hung rosary
An Italian and Irish custom, still very much alive: hang a rosary or a string of beads on a washing line or an outside tree the night before. If you have no rosary, a ribbon tied to the branch is the same gesture in a different dialect.
The buried egg
Bury a whole raw egg at the edge of the ground where the event will be held, at least a week ahead. Say what you are asking for as you cover it. Do not dig it back up.
Old and widespread, and worth clearing with whoever owns the field first.
The turned stone
Find a flat stone on the site. Turn it over, wet side up, and leave it. When the underside has dried, the ground has dried with it — a folk rite and a small piece of practical observation at the same time.
A note on all of these
None of it is a forecast. The sky keeps its own counsel. What these rites reliably do is give you something to do with your hands during the weeks when there is nothing else to do but wait.