Apologies, I thought I included the link. It's the japanese PR release, which is always more detailed:
https://topics.nintendo.co.jp/article/4aca50db-b0bd-4552-827e-9eadda7ce4e9
There is a special section that covers the seasonal celebrations, running from May to July:
Children's Day runs from April 28th to May 5th and has the windsocks and the newspaper hat (both new)
Mother's Day runs from May 1st to May 31st and has the Thank-you Mom mug (old) and the carnation (new)
Cheese Rolling Day runs from May 22nd to May 31st and has the Cheese item (new)
International Children's Day runs from June 1st to June 30th and has the Handmade cape and crow (both old)
Wedding season runs from June 1st to June 30th and has the Nuptial bell, doorplate, ring pillow (all new and with color variants) and the Flower Shower basket thing (new)
Father's Day runs from June 1st to June 30th and has the Thank-you Dad mug (old) and the Thank-you Dad apron (new and with color variants)
Tanko/tango bushi runs from June 5th to June 14th and has the festival zongzi and surichwi tteok (both new, they are food)
Summer/Winter solstices run from June 15th to June 21st and have the corresponding two old items for each hemisphere (although technically, each hemisphere has one new item if they haven't traded for it before- the Sunflower Rug and the Aurora Wall)
Tanabata runs from July 1st to July 7th and has the Bamboo grass (old)
Marine Day runs from July 8th to July 22nd and has something google translate isn't helping and can't find (かじのドアかざり kaji no doa kazari) (new)
Bastille Day runs from July 10th to July 20th and has a phrygian cap (new and with color variants)
Cowboy Festival runs from July 15th to August 15th and has the Rodeo-style spring horse (old)