This is another tedious chinese custom for those married couples (ie: married before 30 years old in lunar calendar):
Your 31 years old birthday celebration (in lunar calendar) have to be organized and of coursed paid by your mom in law (only applicable to men). And for the ladies, your mother (not your mother in law) have to organize the same celebration for you and of course ... all the cost shall be borne by your mother as well.. hooray ..
Apart from that, there are few things that wife's mother need to prepare:
1 set of clothes (pants + shirt)
1 tray of eggs
1 bottle of cognac/wine
2 pieces of fatt kou
FYI, the above items is for 1 person.
If your birthday and your wife's birthday are close to each other, you can opt to hold the celebration together, ie: help your mom-in-law to save cost on 1 luncheon/dinner instead of having 2 luncheon/dinner.
Happy go lucky.