Jehovah's Witnesses have been known to celebrate wedding anniversaries, anniversaries of special events, graduations, etc. Jehovah's Witnesses love to have fun and desire to have fun just like everyone else. The following articles provide just a few examples:
Cross-Country Skiing - Is It for You?
Do You Spend Time With Your Family?
Healthy Fun ON TWO WHEELS
As for an actual observance, Jehovah's Witnesses do observe the Memorial of Christ’s death as true Christians were commanded to observe by Jesus Christ himself. (1 Cor. 11:24-26)
For more, see
The Lord’s Evening Meal—An Observance That Honors God
However, true christians who wish to show exclusive devotion to God strive to avoid ceremonies or customs that may have pagan origins. And basically, most holidays have pagan origins.
Exodus 23:13 shows how exclusive the worship of God must be:
"Be careful to do everything I have said to you. Do not invoke the names of other gods; do not let them be heard on your lips." (NIVSB)
So if pagan ceremonies, customs, god names, etc. are mixed in with ceremonies, customs, etc. that we use today, they are detestable to God. We must completely get away from them:
"Therefore come out from them and be separate, says the Lord. Touch no unclean thing, and I will receive you." - 2 Cor. 6:17
For more, see
Beware of Customs That Displease God