I have been searching the internet and, so far, have not found any account that agrees with the statement that it "always used to be the nearest to 25th March, the annunciation ...". There are several that point out that this year it happens to be the nearest to the feast of the Annunciation. It is bound to happen now and then.

I also found this:
"The French celebrate Mother's Day on the last Sunday in May, where a family dinner is the norm, and traditionally the mother being honoured is presented with a cake that looks like a bouquet of flowers."
and:
"Mother's Day in Spain is celebrated on December 8th. Spaniards pay tribute not only to their own mothers on this day, but also to the Virgin Mary. The day includes religious celebrations across the country."