It really depends on the child. My first DD always wore the size that corresponded to her age, almost down to the day. She still does - she is almost 8yo and wears a 7-8. My DD2, however... she was always in one size larger than her age, until recently. Last fall I packed away all her 18-month size clothes as she was outgrowing them. Now I had to take them all back out, as the adorable new 24-month size outfits for her are too big, she keeps losing her pants. So I would definitely wait, and when the time comes, if the weather on the trip is warmer than weather at home, just buy some appropriate outfits at consignment, goodwill or garage sales specifically for the trip. Then you won't feel bad about buying all these new things when the baby outgrows them before the weather at home warms up!