https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(WMF)#W...
The actual fundraiser will come at the end of the year, when the banners will follow everyone visiting Wikipedia around for over a month (29th of November – 31st of December).
https://meta.wikimedia.org/wiki/Fundraising#Current_fundrais...
Your donors often feel generous as the giving or holiday season approaches. Many will also be looking to make year-end donations for tax relief." [0]
If you donate in March, when no ads are running, that will lead to the ads being seen as less effective.