As of v1.10.0, Dressed to Spell works like this
"Dressed To Spell" is a special event that is playable on the 20th day of each month. There are multiple tiers to complete. All tiers except the first tier have a clothing requirement. You no longer have to wear the required articles of clothing; owning them is now sufficient.
Each month you should always be able to play at least one tier of the event (which may be the same tier you stopped on the previous month if you aren't yet qualified for the next tier). In addition, if you beat a tier and meet the requirements for the next tier, you're allowed to play the next tier--there is no more "one tier per month" restriction.
Prior to v1.10.0, Dressed to Spell worked like this
"Dressed To Spell" is a special event that is playable once a month (currently on the 20th of each month), which has clothing requirements for your avatar bear in order to play. There are multiple tiers to complete, but only one tier can be played each month. This means that if you beat the first tier of the event on March 20th, you won't be able to play the second tier until April 20th, the third tier until May 20th, and so on.
Note that it is insufficient to simply own the clothes required for the tier; your avatar must also be wearing the required clothes in order to play the tier.