Arch on a windy walk way

Not without a lot more information. An arch can be as simple as two vases of tall curly willow arching towards each other and meeting at a central point, to being a real structure. What are you envisioning, and what is this arch supposed to withstand in terms of wind? Pics of the area and the envisioned arch would help.

I just looked at your wedding date...ummm, pretty close for such a substantial DIY project.

A lot depends on the structure of the arch and what material it will be standing on. If it is on earth or lawn, you can use tent pegs to anchor it. On pavement or cement, weigh it down with sandbags which you can cover with fabric, moss, sod etc.