It all depends what you want to do to celebrate.
Here are some really inexpensive (or no cost) things that we have done:
Eat whatever you would normally have for dinner, but use the ‘special’ crockery, cutlery, tablecloth or whatever else you use for a special meal.
Again, your regular dinner, eaten by candlelight - beans on toast eaten by candlelight makes something special of a very basic meal.

Have your meal as a picnic (there is no need to go to a special place for this, just out in your yard will do fine).
Talk with your girls about how special the day was for both of you. The year we did this my girls produced, spontaneously, the cutest cards of what they thought we looked like on our wedding day.
Do something for each other that makes them feel special; here I’m thinking about things like a back rub, foot rub, making a cup of that special tea/coffee.
Take some time out of the busy day just to be with each other, that cup of tea/coffee sitting outside.
Where I’m heading with this is that I think both days should be celebrated, but the form of celebration does not have to cost anything, or could cost very little - in terms of $, in terms of time spent marking the day as special a huge investment.