I know that when a relationship is developing growing, one needs to start finding out the partner's potential light grey, dark grey and black zones about feelings.
It's bad news if the relationship crosses some crucial thresholds and you're living together with financial arrangements, household arrangements, etc ... then, OUT OF THE BLUE comes an issue like Mothers' Day.
It's never nice when it could be the big hill on which to fight a battle of wills and opinions.
Should've passed muster in the wooing/developing period.
My mother always said she didn't care about mothers' day. She always said she just wanted he four kids not to give one another "grief" and to let the day go smoothly. But, deeper down, I gleaned that she half-respected a day for mothers and very much respected the fact that I, from 13yo made an effort every year to make or do or give something to her. A special lunch, a piece of jewellery, a special recipe or cake shared at home, a voucher to one of her favourite clothing shops. It's just on two years since she passed and my three siblings were quite amazed that she'd made notes about almost all my gifts to her (xmas, birthday, mothers' day) over 44 years. It was only after she'd died that I realised I'd left a bigger imprint on her.